Tuesday, 21 October 2014

Technology focus - Gasoline engine technology trends


Engine technology developments are moving at a rapid pace! The main driver being ever more stringent emissions regulations, in combination with market expectations – no-one is going to buy a car with less performance due to emissions compliance! This is a tough challenge for any vehicle OEM but it is promoting an exciting breeding ground for the latest developments in engine and vehicle technology. So, what are these likely to be?...


Smaller engines with higher power outputs are already a fast moving trend (downsizing), this trend will continue - with almost unbelievable targets. Development goals of gasoline engines producing 200kW/litre are being considered by OEM's – the biggest challenge here being durability – that is, how to make such a powerful engine last in production. These gasoline engines could be classified into lower and higher power engine design catagories – with the break point between them being around 180kW/litre. For lower power densities, the main enabling technologies will be the adoption of Miller cycling and cooled EGR, in conjunction with cylinder deactivation and variable compression – although this latter technology is expensive! 

Figure 1 - Saab variable compression concept


In addition, Miller cycling requires specific design attributes for the inlet air path, valves and combustion chamber – reason being, to promote strong tumble in the incoming charge as this ensures enough charge motion to provide good turbulence in the mixture, for rapid flame growth – that would otherwise be compromised due to the late closing of the inlet valve.

Figure 2- Miller cycling

New spark plug electrode designs are under development in order to produce turbulent jets of burning mixture into the combustion chamber, as opposed to just an electrical arc to provide an ignition source. Laser ignition has shown very promising results, however, at the moment this technology is still confined to the laboratory as it is expensive and physically too large for production applications.


Figure 3 - Electric cylinder charging systems will become the norm.

For higher power densities (likely to be adopted in performance vehicles) variable cylinder charging and porting concepts will be used in addition to the above developments. Multiple charging systems (turbo, super and e-booster) will be used to employ charging and evacuating of the cylinder. Dual injection systems will be used, with port injection for use at high-load conditions, to reduce particle emissions.

Figure 4- Dual Injection systems to reduce PM emissions from gasoline engines

For very high performance engines, in order to provide the required durability, it is likely that control system strategies will be used to ‘limit’ engine power below normal operating temperatures (cold start and running). This restriction will allow an increased safety margin for engine components - promoting longer engine life. In addition, these strategies could include the adoption of a ‘spark plug cleaning’ mode – to ensure that a high performance vehicle that is not driven with sufficient load/speed regularly, will not break down due to fouled spark plugs. (similar in concept to DPF regeneration).

Thursday, 17 July 2014

Engine Combustion - Compression Ignition (Diesel)



In a diesel or compression ignition engine, the first and major difference compared to a spark ignition engine is the way that fuel and air is prepared for combustion, also, the way combustion is initiated. 


Diesel engines induce air only during the intake stroke - the air charge is compressed in the cylinder, heating it accordingly, the final temperature at the end of the compression stroke is above the self ignition temperature of the fuel and this factor is essential, as this initiates the combustion event when the fuel meets with the hot air. The advantage of compressing air only is that we don't have to consider self ignition of any fuel/air during compression (as per gasoline engine) as at this point in the engine cycle, there is no fuel to burn! The combustion process is quite different to the gasoline engine, the timing and rate of combustion is controlled via the introduction of fuel into the cylinder (via the fuel injection system). The combustion process itself takes place at the interface between the fuel and air. Therefore, sufficient air motion in the cylinder (generally swirl in a diesel engine) is essential to sweep away the products of combustion, ensuring that the fuel charge always has sufficient oxygen at the flame interface to prevent to formation of soot due to localised oxygen starvation. 


Fig 1 - Air motion in a diesel engine is generally 'swirl'

The overall volume of the combustion chamber itself has a variable air/fuel ratio during operation, that is only chemically correct at the fuel to air interface. In most operating conditions, the average air/fuel ratio in the cylinder is considerably weak (compared to stoichiometric). The engine power output is controlled by the amount of fuel injected, so no throttling is needed and this improves efficiency at part load due to the lack of pumping losses associated with restricting the airflow into the engine. The technical term associated with diesel type combustion is ‘diffusion’ combustion, as the fuel burning takes place at the interface where fuel diffuses into the air, and vice-versa. 

Due to the fact that fuel and air have to be mixed during the compression/expansion cycle (as opposed to pre-mixed, outside the cylinder) this reduces the amount of time available to complete the whole mixing and combustion process. Hence, generally speaking, diesel engines cannot rev as highly as gasoline engine. Therefore, to get more power from a diesel engine you increase the torque by turbocharging it! - common practice these days. It’s notable though that the diesel engine combustion cycle, and engine itself, is more efficient than gasoline for several reasons - the higher compression ratio increases the cycle efficiency, the lack of a throttle reduces pumping losses and the high precision, metered injection system reduces cylinder-to-cylinder variation.




Fig 2 - A common rail diesel fuel injection (FIE) system

Diesel engines have undergone considerable development over the last few years, mainly in the area of fuel injection system technology. These developments have enabled sophisticated, electronically controlled injection systems, that can help reduce particulate emissions as well a engine noise emissions. I think that anybody would agree that travelling in a modern diesel engine car is no longer a noisy or unpleasant experience. Modern diesels are very refined and smooth in operation!



Fig 3 - Direct and Indirect fuel injection - direct injection is predominant now!

All modern diesel engines for passenger cars use direct injection technology (as opposed to indirect). In the past, indirect injection - injecting fuel into a pre-chamber - was technology used to create the required air charge motion to speed up the combustion event, thus increasing the maximum possible engine speed and power density. However, the increased surface area of the combustion and pre-chamber increases heat losses and reduces efficiency and has now been completely superseded by direct injection systems for most applications. In a modern diesel engine, the fuel injector nozzle sprays a complex, engineered spray pattern into the hot , highly turbulent combustion chamber gases, to initiate the combustion event at around TDC.  The fuel is injected radially into the combustion chamber, the liquid fuel vaporises and mixes with the air as it travels away from the injector tip nozzles. The fuel self-ignites at multiple ignition sites along each of the injection sprays. 


Fig 4 - Diesel spray pattern and combustion from a thermal image system


The design of the combustion chamber, in the piston bowl, is critical to the efficiency of the combustion event. This design creates the necessary motion and energy in the cylinder charge to make sure that each tiny droplet of fuel has sufficient oxygen for complete combustion, right throughout the injection period. 


Fig 5 - The 3 phases of diesel combustion

The initial combustion takes a certain time period to establish, known as the delay time, then the fuel will auto-ignite creating a very rapid energy release and the flame spreads rapidly through the fuel that is exposed to sufficient air for combustion. This creates a rapid rise in cylinder pressure, forcing the piston down the cylinder. As the power (or expansion) stroke continues, further mixing of fuel and air occurs, accompanied by further, more controlled combustion period where energy release is controlled by injection rate. Note that it is the rapid release of energy, after the delay period, which causes the characteristic combustion ‘knock’ associated with diesel engine.



Fig 6 - Common rail, electronic diesel systems allow multiple injection events with better control of the combustion process

Modern, electronic fuel injection systems, with multiple injection events, effectively reduce this noise via a more gradual introduction of the fuel into the cylinder (via pre-injection events) as opposed to a single-shot event, where all fuel is injected at once (causing rapid pressure rise and noise). Note that single-shot injection strategies were all that was possible with a simple rotary or in-line injector pump in the past. In summary, the key points to consider with respect to the compression ignition engine are:
  • The fuel/air mixture is prepared internally in the cylinder, during the engine cycle and relies on self ignition
  • The engine power is controlled via the quantity of fuel injected in each engine cycle. 
  • The compression ratio is not limited by the fuel as the compressed charge is just air, It is only limited by the strength of the engine design as peak cylinder pressures are very high
  • In operation, engine maximum torque is limited by peak pressures/mechanical loading
  • Rapid pressure rise, generated by the self-ignition of the fuel, creates the diesel engine noise

Saturday, 10 May 2014

Combustion Pressure measurement for efficient Engine Diagnostics

Measuring cylinder pressure in an engine, in order to establish the combustion efficiency and losses is a well-established, widely used technique. In fact, it goes back to the days of steam engines! 

The in-cylinder pressure, with respect to crank angle is important for understanding the rate of energy release, and to be able to understand the amount of work done in the cylinder prior to transfer to the crankshaft, allowing losses to be established. However, these measurements normally require sophisticated equipment with specialised sensors. In addition, the engine normally requires some level of modification or adaption in order to be able to access the required measured parameters (cylinder pressure and crank angle). For these reasons, cylinder pressure measurements are normally the reserve of research and development environments.


 Figure 1 - Cylinder pressure plotted against volume giving the classic diagram from which the Indicated mean effective pressure is derived



Figure 2 - Pressure vs. crank angle - gasoline engine running in knock condition


In theory though, measuring pressure in the cylinder for diagnostics is quite feasible these days, this is due to the reasonable cost of high speed measurement and recording equipment available to the after-market for sensor and actuator signal measurements (e.g. oscilloscopes). These are normally applied specifically for fault diagnosis of vehicle electronic systems, but these devices are easily capable of measuring a signal from a cylinder pressure sensor, of a suitable type, installed in the engine cylinder.

Another more recent development is the availability of sensor technology of appropriate durability, with scalable output ranges, that come with appropriate non-intrusive adaptors which allow the sensor to be installed into the cylinder in place of the spark plug. This technology has facilitated a trend towards examining pressure traces, in diagnostic procedures, in order to reduce the amount of time spent getting to the root cause of difficult to trace faults, especially those which generate non-specific or misleading fault codes.

But why does the cylinder pressure trace help us? and what are we actually looking at? Also, how we can interpret the data effectively to make good diagnostic judgements. A good place to start is the system set-up…


Figure 3 - Overview of system configuration for cylinder pressure measurement (Source: LHM Engineering)


The diagram above provides a system overview. Basically, the scope hardware is connected to a PC and this is the data acquisition system. The transducer is remotely mounted (from the cylinder) and produces an analogue voltage in response to the pressure applied to it. This pressure comes from the combustion chamber via a pipe and adaptor which takes the place of the spark plug. The diagram below shows an actual installation ready for measurement. Note that the target cylinder must be ‘disabled’. Normally this can be achieved by disconnecting the electrical connector to the fuel injector (where 1 cylinder is to be disabled – running test). For a cranking test, the CPS (Crankshaft position sensor) should be disconnected which prevents starting of the engine in full (normally no fuel or spark).


Figure 4 - A typical installed sensor, connected to the engine via a pipe/spark plug adaptor (Source: LHM Engineering)

For this type of measurement, we have to consider the boundary conditions as there are 2 main limiting factors to consider with respect to the acquired data for diagnostics.

1. We have to remove the spark plug and measure the motored pressure curve - so we need to motor the engine - either with the other cylinders firing, or via motoring with the starter motor. This provides a motored pressure curve from which much information about engine health and general condition can be gained. However, it's clear that firing is not possible and hence the engine cylinder is not working under it's true thermal and loaded conditions

2. The data is sampled in the time domain - that is, the scope will sample with a regular sample rate with respect to time, not engine position (although this can be derived subsequently). Time based sampling alone means that the engine position and cylinder volume can only be estimated from the raw data. Hence accurate calculations that would involve cylinder volume are not really possible (they would be too inaccurate), an example of these calculations would be the Indicated mean effective pressure (IMEP) which gives a measure of the work done in each engine cycle. Due to this, the data which is measured is only suitable for calculating direct results - those which are derived from the raw data alone - for example, the peak pressure value, or the rate of pressure rise.

However, the motored curve can be extremely useful and can tell us a lot about the general condition of the engine. When motoring, the engine cylinder it effectively becomes a simple air compressor and expander (rapid compression machine), of course, this is not a very useful type of machine but by examining the pressure curve, we can establish an idea regarding how efficient the engine behaves in this mode of operation, and that's important because in between the compression and expansion part of the engine cycle, the four stroke engine is effectively a pump, expelling the burnt gases and drawing in the fresh charge, this part of the engine cycle, known as the gas exchange, is essential for efficient and effective combustion - optimising this part of the engine cycle is of high interest to engine developers. It worth noting that most instability and variation is related to the combustion event itself - when measuring a motored cylinder, there is none of the errors or variation relating to combustion, therefore the repeatability of the motored curve is excellent and there are a number of useful metrics that can be derived from this curve to assist diagnostics

Curve analysis
Let’s look at the raw pressure curve with respect to the motored engine cycle. The diagram shows the full cycle, with phase markers:


Figure 5 - A full engine cycle, separated into each engine stroke phase with markers (Source: LHM Engineering)


You can see clearly each of the four strokes. Compression and expansion during the motored curve is wasted energy - some of the energy is converted to heat in the process and subsequently lost (rejected to the surrounding engine thermal mass) - there is no work done but we can establish the peak cylinder pressure from this curve as a metric for general engine condtion. It’s obvious that any significant cylinder leakage (via the piston rings, head gasket or valves) will reduce the peak value generated, this will be obvious in a cylinder to cylinder comparison - however, the whole cycle curve can give us much more information about the possible reasons for reduced compression, as opposed to just indicating that reduced compression exists


Figure 6 - Full cycle pressure curve


Note - The compression and expansion ratios are design factors of the engine optimised to give the best possible efficiency from the engine and combustion system design - therefore a loss of compression due to worn components gives a considerable loss in engine cycle efficiency



Figure 7 - The high pressure part of a motored cycle - this should be presented as a nice, 
smooth curve with good symmetry – in this diagram, two completely separate measurements on different engines are overlaid - both curves can be considered as showing a 'good' condition engine

Relative loss of compression pressure is not just due to leakage factors - it could also be due to engine throttling or inefficient breathing due to worn valve gear components or incorrect valve timing or clearances. These breathing problems normally impact on the pressure curve dynamics. You can see from a typical measured curve that there are resonance effects during the gas exchange part of the cycle. 


Figure 8 - Similar to the above diagram, but focussed on the gas exchange part of the cycle, as before though, the measurements are very similar in form. The resonance during gas exchange can clearly be seen on both curves - this diagram shows that even different engine display similar, common characteristics on the motored curve


When using a remote sensor (i.e. a sensor connected to the engine via a pipe). It is likely that oscillations are generated due to the air passage between actual the sensor membrane and the in-cylinder air volume. However, these flow dynamics should not vary significantly between cylinders on the same engine - as the sensor and pipe, as well as the cylinders should all be the same (more or less) with respect to dimensions and physical properties. Therefore, any small difference on the curves will be due to the flows within each cylinder and can thus be used for diagnostics. In particular, it is worth studying the baseline of the pressure curve, plus the amplitude and frequency of the resonance. However, try to be sure that when making measurements between cylinders for comparison, that the cylinder conditions are as similar as possible, in particular with respect to engine speed and cylinder temperature during the measurement.


Figure 9 - Pressure sensor installation directly into the cylinder for R&D measurement applications



Figure 10 - Two separate measurements compared - one made using a scope and remote sensor (lower), one using an in-cylinder, direct installed pressure sensor (upper) - you can see much less pressure wave resonance where the sensor has no connecting pipe, as in the latter case - however, the pressure wave dynamics can still be useful in diagnostic procedures

In addition to high–pressure measurements (i.e. within the cylinder) a useful approach is to monitor low pressure effects – specifically, in the exhaust and inlet. With a suitably calibrated sensor, the pressure dynamics, pre and post combustion chamber, can be easily gained and are useful to help on the diagnostic pathway! In terms of the diagnostic process. It is very worthwhile to try and measure the low pressure effects first, as installing the sensor for this task is easier and less effort - this helps to gain some insight to the root cause of a problem with lower initial effort. The low pressure dynamics can also highlight breathing issues and flow issues, in addition, by measuring other signals and using them a phase markers  (for example, a cylinder specific ignition pulse), cylinder specific related  issues can often be identified.




Figure 11 - Inlet and exhaust measurements - in this case highlighting a problem with a specific cylinder (Source: Pico Automotive)

The diagram above below comes from a diagnostic procedure where a cylinder misfire was apparent but the root because not that clear. In this case the manifold pressure and exhaust pressure were measured and as you can see from the inlet trace, a cylinder specific issue could be seen on the signal. This allows the diagnostic technician to know that there was a problem with one of the  cylinders, with the breathing on the inlet side. The root cause in the end being a valve clearance issue. There are similar case studies in the public domain that highlight the value of using pressure measurement to support diagnostic studies looking for classical mechanical faults, which cause an electronic failure mode or warning via OBD system (the OBD system is often considered to be able to identify electronic related failures only - often though the root cause can be a mechanical issue).

Summary
In conclusion, its clear that pressure measurement can support efficient diagnostics, whether the failure is electronic or mechanical. The equipment available for this measurement technique is now easily available and reasonably priced. If you buy the kit, keep it handy in the workshop and practice making measurements on a regular basis. You will build up knowledge and be confident to carry out the process whenever needed. In addition, regular use can shorten diagnostic time, increase efficiency and shorten return-on-investment time after purchasing the kit.

However, you will still need a well-defined process to support your diagnostics in this specific area. A suggested approach, using a Picoscope or similar could be:

1. Collect, identify and clear any fault codes

2. Carry out a compression test to establish mechanical health as an initial test – ideally using an non-intrusive method – note any cylinder specific effects or deviations greater than 10% between cylinders

3. Measure inlet pressure and examine closely the dynamics – using a reference pulse check if any cylinder specific issues correlate with the compression test data

4. Measure the exhaust pressure and pulse – check dynamics as above

5. If a deficient cylinder is identified, instrument the cylinder with pressure sensor and measure some traces (disable cylinder firing). Analyse raw curves

6. If in any doubt, measure some pressure curves from another cylinder for comparison during analysis – compare peak pressure values, plus the pressure wave dynamics during gas exchange – in particular pressure pulses resonances and equalisation ramps


Following this process should take you from the least intrusive method, through the more involved procedures, but in the right order. So that if you uncover the problem ‘on the way’ then you don’t need to proceed further - unless of course you have the time to spend to validate your findings! This approach will ensure that you get to the root cause a quickly as possible - ensuring an efficient process and a 'fast-time-to-find-fault'.

More information
For more background, take a look at the information below:


Tuesday, 22 April 2014

Technology Focus - Morgan Cars: The high-technology future of the Classic British Sports Cars

A recent visit to Morgan Cars prompted to write this article. I was really interested to see how the mix of classic heritage skills is being blended with high-tech Engineering to produce really great, desirable cars. This is a picture you can see mirrored at other strong British automotive brands (JLR, Aston Martin, Bentley Motors etc.).


Morgan Cars is a high value brand, associated with the traditional, high quality craft skills needed to create a classic British sports car - one that creates excitement and enthusiasm for the driver. Thus, buying and owning a Morgan is a really special and personal experience, knowing that you have invested in a vehicle that has been designed, created, Engineered and manufactured with exceptionally high precision and care. This has been the Morgan tradition and hallmark for many years!

Times are changing though, legislation in relation to safety and exhaust emissions are the main drivers for technological developments in Automotive Engineering. Customer expectations are high with respect to performance, drivability and emission compliance - and Morgan has no exemption here! So the question is - what is Morgan doing to meet these challenges. The answer is that Morgan is investigating a number of technologies to investigate and meet future challenges. A considerable undertaking when you consider that one of the constraints is to retain the heritage and tradition of the brand and the marque!

LIGHTWEIGHTING
In general, light weighting concepts have a number of benefits - advanced materials have superior stiffness, providing improved handling chassis. The lighter overall weight reduces inertia - improving acceleration and cornering performance. However, a real benefit in fuel consumption (and reduced emissions) can be gained by reducing vehicle mass as much as possible (whilst maintaining structural integrity). Morgan has successfully experimented with magnesium for body structures - which is the lightest structural metal available (30% less dense than aluminium). The use of sheet magnesium for vehicle structural applications requires hot-forming, increasingly being adopted by premium car manufacturers, as this process can produce large, complex body panels. Morgan intends to adopt the newly developed technologies (produced by an experimental project) on its next generation of premium sports cars.


Magnesium has significant benefits for manufacturing car body panels – mainly its strength combined with light weight, to reduce vehicle mass

ELECTRIFICATION
This is a general term used in Automotive Engineering covering numerous applications of applying electric drives and motors in order to provide power for accessories, or traction – but only when needed (for example electric power steering). Morgan has experimented with the option of a full electric power train, a project known as the Plus-E. An electric sports car with a five-speed manual gearbox, designed by Morgan with the support of British technology specialists Zytek and Radshape. This was developed as a concept vehicle to test market reaction, but the radical new roadster could enter production if there is sufficient demand.



The Plus-E electric concept vehicle – could be coming to a Morgan showroom near you soon


This vehicle combines Morgan’s traditional look with high-technology construction and a power train that delivers substantial torque - instantly at any speed! This is combined with a manual gearbox to increase both touring range and driver engagement. The Plus E is based on an adapted version of Morgan’s lightweight aluminium platform chassis with power provided by a new derivative of Zytek’s 70kW (94bhp) 300Nm electric machine (already well proven). The power unit is mounted in the transmission tunnel and drives the rear wheels through a conventional five-speed manual gearbox. However, the system has sophisticated electronic controls to synchronise the motor speed and torque during shifts, to provide a seamless gear change with minimal interruption of traction for a perfect gear shift. The combination of multi-speed transmission and high torque e-machine allows operation of the motor at maximum efficiency, for as much of the time as possible, whilst providing the best possible performance for the driver experience. The project is future oriented and encompasses the exploration of alternative transmission types (CVT, DSG) as well as different battery chemistry options.

ENGINE TECHNOLOGY
Morgan employs state-of-the-art Engines, supplied by leading Automotive Manufacturers. These power units are integrated into the overall Morgan chassis and then calibrated to adapt them to the unique character of the Morgan vehicle. The power units are selected specifically to incorporate the latest technologies for emissions reduction and engine efficiency. For example, the V8 power unit employs direct injection - this technology improves efficiency at part load due to the fact that no throttling of the engine is needed to control power output (it's controlled by injected fuel quantity). In addition, knock resistance (knock is a limiting factor for the efficiency of a gasoline engine) is improved by advanced fuel injection systems that use high pressures, to provide a well prepared fuel/air mixture - this is advanced technology but, the current state of the art engines now include downsizing or down-speeding concepts in order to reduce friction losses and operate the engine at maximum efficiency for as much of the time as possible. Engines with high specific power outputs, based on turbo-charged/boosted concepts, are expected to dramatically increase their market share within the next five years. Further down the pipeline, engines will evolve again to meet ever changing and more challenging targets - technologies such as Variable valve lift, Variable compression ratio and variable ancillary drive systems (oil and coolant pumps) will become mainstream, in addition to energy recovery (thermal and kinetic - as used in Formula 1 from this year) - this area is very promising technology to improve overall power train efficiency.




Energy recovery – Formula 1 technology that could be used by Morgan cars to improve the efficiency of the overall power train. Thermal heat recovery is also applicable (now applied in Formula 1 cars)

TRANSMISSION TECHNOLOGY

The transmission and the engine have to be considered and optimised together in order to provide a harmonised power unit that delivers the performance expected by the driver, combined with meeting legislative demands. Manual transmissions are common with 5 or 6 ratios. However, in the near future, more ratios are needed, in conjunction with automation of shifting and control, in order to keep the engine operating in the optimum fuel consumption range. It is suggested that 10 speed transmissions will be needed and common place (in DSG form). This could be combined with an electric machine for low power requirements at low speed. Electrified transmissions with electronic control can be used to reduce fuel consumption and emissions in several ways - The e-machine can provide power at low or zero speed where the combustion engine is very inefficient. The shift control strategy can be combined with engine control to give the optimum shift point for maximum efficiency. Also, the electric machine can be used to provide seamless shifting and constant tractive power. There are a number of transmission concepts available and in use, but no clear leader. If integrated into a Morgan Cars power train, the transmission concept chosen will have to support the performance and driveability that matches the marque!



The Eva GT – tomorrows Morgan available today! High technology, advanced design, stunningly attractive!

OTHER DEVELOPMENTS
A combination of future technologies has been combined in the Morgan life car project. This prototype received a rapturous response and according to some sources, Morgan has decided to take it from a prototype to a fully-fledged production vehicle. There have been some changes to the original brief, making the car more practical, while retaining the revolutionary features that made LIFE car unique.

The proposed vehicle now includes a super-efficient, series hybrid drive train, developed using some of the country's best universities, making use of the wealth of knowledge in their research departments. The drive train will power a vehicle that epitomises Morgan core value of innovation. The use of sustainable lightweight materials will ensure that not only is the vehicle fuel efficient, with a low carbon output, but that at the end of its very long life, it will be easily recyclable. The goals set are for a vehicle
  • 1000 mile range
  • Ultra lightweight (sub 800kg)
  • 15 mile EV range
  • 0-60mph in 7 seconds
  • ~£40,000 Price
The Morgan life car project – Next generation of Morgan sports car combing light weighting with an advanced powertrain.

SUMMARY

There is no single technology that will secure the future for Morgan, or any other manufacturer. Even the mainstream manufacturers are gambling with a combination of low carbon technologies in order to meet or achieve current and forthcoming requirements. It could be considered that Morgan cars, as a manufacturer of 'niche' vehicles does not need to lead but just follow industry trends. However, that is not the Morgan way! Even though production volumes are low (compared to the mass market), innovation and technology are within the Morgan DNA. As the only remaining, true British manufacturer, Morgan takes its responsibility to be a leader very seriously. A clear example of this is the position Morgan takes in this area, with many research projects and collaborations with leading universities, who can undertake the research task and produce tangible technology that can be ported into production by Morgan.

There is no doubt - Morgan is a leader in pushing the boundaries of design and technology for Classic British sports cars, and will continue to do so for many forthcoming generations.

Monday, 14 April 2014

Technology focus - Future fuel injection technology for Common Rail diesels - Intelligent injectors

In this feature we're looking at some interesting developments in fuel system technology for common rail diesels. In previous posts, we've looked the pressure wave phenomena in common rail diesels and how this can significantly affect the accuracy with respect to the quantity of injected fuel per stroke.

Denso has addressed pressure wave phenomena and taken the intelligence in diesel engine fuel systems to the next level with the introduction of their Intelligent Accuracy Refinement Technology (i-ART). The technology features a fuel-pressure sensor with an integrated microcomputer which monitors injection pressure, based on various input data. The whole assembly is integrated into the top of each fuel injector. The closed-loop system precisely manages injections of fuel to match specific drive cycle conditions. It replaces the single pressure sensor typically positioned in the fuel rail. Denso engineers have stated that i-ART can improve fuel efficiency by 2%, compared with open-loop systems. It was developed to enable diesel engines to meet Euro 6 regulations with a reduced after-treatment burden. Toyota also is using i-ART systems in upcoming 3.0-L commercial diesel engines.



Fig 1 - The new range of Volvo power units includes Denso i-ART technology (source: Volvo)


A conventional injection system could only detect an injection quantity based on indirect methods such as combustion or an engine rotation fluctuation. The i-ART system enables a direct detection of the injection quantity, each injector is equipped with a built-in fuel pressure sensor to measure injection pressure inside the injector itself. Based on the information from the built-in pressure sensor, the Engine Control Unit (ECU) reads fuel pressure values for each injection rapidly, and calculates an actual injection quantity and timing for each cycle, based on this information, using a rapid waveform processing technique. The learning value for the injection quantity and timing calculated with the i-ART system are applied to subsequent injections and adapted throughout its lifetime.


Fig 2 - System overview - i-ART intelligent injectors and feedback data flow (source: Denso)

The actual pressure wave form generated by the i-ART pressure sensor is shown in Figure 3. The system performs a pre-processing by compensation to the non-injection  pressure waveform in order to estimate the injection quantity and timing correctly. It then calculates the injection rate based on the processed pressure waveform which is optimised by filtering. The injection rate can be expressed by five parameters of a trapezoid shape. Calculating the area of the trapezoid, the injection quantity is obtained. (Figure 3 lower diagram) 

The i-ART system learns the injection quantity and timing constantly while the engine is in operation - there are two advantages to using this characteristic. The first is the possibility to use a triple pilot injection strategy - which allows a lower a compression ratio to be used, as less heat is needed to be able to ignite the fuel under all operating conditions. This is due to the improved mixture formation which promotes efficiency in the early stages of fuel injection/initial burning. In addition, this allows a sufficient preheating effect for the fuel with a reduced overall cylinder temperature, such that NOx and PM can be reduced. As a second advantage, in conjunction with cetane number detection, a stable combustion with minimised combustion noise can be achieved irrespective of the variation of cetane number with fuels in certain markets.






























Figure 3 - Fuel pressure waveforms at the i-ART injector (source: Denso)

This technology is a big leap for common rail diesels, but also a significant step forward for measurement technology that can now be employed in production. There are significant advantages to being able to establish the fuel pressure directly at each injector, at the point of injection, as this helps considerably in being able to model the injection rate and fuel mass per stroke. The ultimate goal is to develop an injector where the rate and quantity of injection can be varied without a step and within a cycle. This would then facilitate the ability to truly control the combustion and energy release in a diesel engine, with high precision, on a cycle-by-cycle basis. I wonder who will get there first - Bosch, Denso, Continental, or someone else....assuming they haven't already!

Tuesday, 18 March 2014

Liquid fuels for internal combustion

It’s probably clear from technology pathway, that we are heading towards an ultimate goal of us all driving fully electrical vehicles, fuelled by electricity from carbon free generation – probably generated mostly from nuclear power. Although, it’s not that long ago since we were all rejecting Nuclear power due to the associated problems of disposal of nuclear waste and risks of contamination. Oh well – what goes around comes around!


Nevertheless, this seems to be the direction all the manufacturers are heading for, It’s been proven that electric cars can have the performance and driveability that modern drivers need. If the driving range can be extended and the fuelling infrastructure issue solved, then we’re nearly there. However these two points are massive technological barriers at the moment! Battery capacity is currently limited by physics. Until the next major development step in battery chemistry, range will be limited and driver range anxiety will be a major issue to overcome.

Figure 1 - Battery vehicle on charge

Even if battery capacity and range can be extended, the fuelling issue remains a problem for full electric vehicles. If you think about it, your current vehicle carries a store of energy in the chemistry of the fuel compounds. In a few minutes you can refill the car with a massive amount of energy, enough to propel the car for hundreds of miles. Assuming that you had a battery powered vehicle that could store enough electrical energy for the same distance, the power cables and electrical infrastructure needed to fully charge the battery from empty, in the same few minutes, would be impractical. The cables would have to be so large in cross section they would be impossible to lift. The current needed would be massive, requiring a very expensive system of cables, switchgear and transformers that would not be feasible in use.

Of course, these technical hurdles will be overcome in due course, however the main impact for the immediate future is that, in combination with other technologies for low carbon, liquid fuel and combustion engines remain the most practical source of power for personal transport. Mainly due to the ease of storage of large amounts of energy in chemical form. In addition, the existing distribution network already in place in the form of liquid fuel stations and garages supplying fuel in this form to storage on-board vehicles.



Figure 2 - The current technical solution for energy storage and replenishment

So, that is the main topic discussed in this blog article – the future of liquid fuels. We’ll look at what fuels are available to make a contribution to reducing carbon emissions and what is the implication of using these fuels. But before that, we’ll look at the main properties of liquid fuels for spark and compression ignition engines, noting what’s important for each type of fuel in it’s application:

Fuel for Spark Ignition
We all tend to think of petrol as the common liquid fuel for spark ignition engines, and this fuel has been used for many years, not so obvious is how it’s changed over the years. Early petrol was quite ‘heavy’ and didn't evaporate very easily. Early engine designs used heat from the engine itself to try and maintain the petrol in vapour form, by running fuel lines close to exhausts for example - which seems ludicrous by today’s standards. Petrol has evolved over time to reduce, and then remove lead content. In addition, there has been a general trend in the reduction of anti-knock quality due to improved engine designs and controls allowing more resistance to knock. Although the quality of petrol varies widely between the differing global market requirements. Lets look at the main properties of petrol:


Figure 3 - Gasoline fuel properties; the trend since 1993 

RON and MON


The most important property of petrol is it’s resistance to Auto-ignition, that is - the anti-knock quality of the fuel. This is characterised by the fuels octane rating - the higher the octane rating the better the fuels resistance to auto-ignition. There are 2 procedures used to determine this property, the Research method (giving the RON - Research Octane Number) and the Motor method (giving the MON - Motor Octane Number). The method of determining the octane rating of a fuel involves the use of a special, research test engine for octane rating - known as a CFR engine (Collaborative Fuel Research).




Figure 4 - A CFR knock rating engine; used in fuel research testing

This engine is a single cylinder, variable compression engine. The test fuel is run in this engine, the compression ratio is altered to give a standard level of knock intensity. This condition is then repeated with reference fuels that contain a mixture of iso-octane and heptane in various proportions. Iso-octane has the highest resistance to auto-ignition and is rated with an octane rating of 100, heptane is rated at zero. By measuring the test fuel for it’s anti-knock property in the CFR engine, this is then compared in the engine to a reference blend of fuel that matches it for anti-knock performance. The ratio of iso-octane to heptane in the reference blend that matches the anti-knock quality of the test fuel defines the octane number i.e. an octane rating of 97 means the fuel has the same octane behaviour in the CFR engine as a blend of 97 % iso-octane and 3% n-heptane when tested.

The RON and MON ratings are different because they are determined under different operating conditions of the test engine. During RON measurements the engine is running at 600 rpm, oil temperature 57°C, coolant temperature 100°C and the inlet air temperature set at 52°C. MON is measured at an engine speed 900 rpm, oil temperature 57°C, coolant temperature 100°C and the intake mixture temperature set at 149°C. This method also uses variable ignition timing. The result is that the MON method places greater demand on the fuel under test due to higher temperatures and heat, such that MON figures are generally lower than RON values. Note that the octane rating of a fuel has no bearing on the chemical energy value contained in the fuel. It is only a measure of the fuel's tendency to burn in a controlled manner, rather than exploding in an uncontrolled manner causing knock.

Volatility
The second most important property for petrol is it’s volatility - that is, how easily does it evaporate and how much heat energy does it need to do this. The reason is that the fuel needs to vaporise easily enough at low temperatures to ensure good mixture formation. But it should not so volatile as to vaporise in the fuel lines at higher temperatures as this would cause hot starting problems. The international standards for fuel properties define a number of parameters to express the fuels volatility, including boiling curve, vapour pressure and vapour lock index.

Other Additives
Petrol can contain other chemical elements, added for various reasons, and these are generally controlled and regulated - these include Sulphur, Lead, Aromatics, Oxygenates and other metallic components. Many fuel suppliers will include various additives in the fuel to improve aspects of engine performance, for example via friction reducing additives, or contamination/deposit removing chemicals (detergents). Corrosion inhibitors and anti-aging additives are common in most petrol fuels.

Fuel for Compression Ignition
As we probably all know, Diesel fuels have to auto-ignite. There is no timed spark or ignition energy source. The liquid fuel is injected into the hot air in the cylinder, where ignition of the fuel must take place as soon as possible in order to start the combustion process efficiently and with minimum delay. In addition, the diesel fuel often has the need to lubricate the fuel injection pumping equipment to provide cooling and prevent seizure, the operating tolerances in the fuel injection system are minute and hence the heat generated and the lubricity required is a considerable additional task for the fuel. Let’s take a look at the important parameters of diesel fuel:

Ignition delay and Cetane
The most important property for diesel fuel is how easily the fuel will auto-ignite and this is expressed as a number known as the Cetane rating. This is a the complete opposite to petrol fuel and the Octane rating and, in fact, the Octane and Cetane rating of a given, distilled fuel have an opposing relationship (i.e. low cetane = high octane etc.).


Figure 5 - Octane/Cetane relationship
The importance of this auto-ignition quality is that fuels with number which have shorter ignition delays provide more time for the fuel combustion process to be completed. Hence, higher speed diesels operate more effectively with higher cetane number fuels and cold starting is easier. In addition, with long ignition delays (ignition delay is the time between start of injection and start of combustion) diesel knock occurs due to the fact that more fuel can be injected into the cylinder before combustion starts, so that when combustion does occur, the greater fuel mass creates a greater pressure rise due to the combustion being too rapid. This creates the characteristic diesel knock noise and associated stresses on engine components.

This ignition delay property is expressed by the Cetane rating and this is derived in a CFR test engine with a similar procedure to the Octane rating. Under standard operating conditions, the engine compression ratio is adjusted to give a defined ignition delay period. Then, reference fuels, consisting of hexadecane (cetane) and isocetane (given cetane ratings of 100 and 0 respectively) in various proportions are run in the engine until the matching blend is found, the quantity of hexadecane in that blend then defines the cetane rating (65% hexadecane gives a 65 cetane rating). A cetane number in the range of 40-50 is typical, and greater than 50 is optimum for modern diesel engines. Another performance metric relating to ignition quality is the Cetane index - this is a calculated value based on the viscosity and density of the fuel, so no test engine is needed for this figure, however it is only accurate for fuel without additives.

Lubricity and Cold Flow
The lubricating property of diesel fuel is important to maintain the correct operation of the fuel injection equipment normally used, the fuel directly circulates in the equipment to provide cooling and lubrication. The lubricating property of the fuel is defined in the European standard (EN590) and is measured using a special high-frequency test rig. Cold flow property is also an essential consideration for diesel fuel, the precipitation of wax crystals at low temperatures causes blockages in the fuel system and generally, diesel fuel in the winter period is charged with additives to prevent this happening. This property is defined as the CFPP or Cold Filter Plugging Point (also defined in the EN590 standard).



Figure 6 - Waxed Diesel!
Viscosity and Density
Viscosity is important to ensure that the fuel moves freely through the injection system, particularly at lower temperatures, in addition, the viscosity can affect the injector spray pattern and fuel/air mixing in the combustion chamber. Diesel engines are also particularly sensitive to fuel density, slight changes in density (even due to operational temperature change) can massively affect the mass of fuel quantity injected per stroke, which in turn, has a dramatic effect on the actual engine power output (as the diesel engine power output is directly controlled via the injected fuel quantity).

Other additives and compounds
Enhancers to improve the lubricity and cetane rating are common. As are detergents to inhibit the formation of deposits on injectors. Anti-foaming additives are incorporated to reduce excessive foaming, thus allowing optimum (reduced) re-fuelling times. Sulphur occurs in diesel fuels, and this is chemically removed in the refining process in order to meet European limits. This is challenging for the producers of fuel as reduced Sulphur results in reduced Lubricity, however, this is offset in modern diesel fuels with the use of lubricating additives.

Summary
We will follow up this blog with another one at a later date - looking at the future prospects for liquid fuels...