Tuesday, 4 June 2019

Technology Focus - Jet Ignition

Mahle is pushing the boundaries of achievable efficiency for spark ignition engines with the use of its Jet Ignition system - which is evolving quickly as a leading technology enabler to reduce emissions even further for next generation of engines. Mahle has already used Jet Ignition combustion concepts successfully in high-performance engines with a different development focus - but impressive results!

Mahle Jet Ignition concept - Injector and spark plug both mounted in a pre-chamber

The available Jet ignition concepts developed by Mahle are classified as passive and active -  Passive using a ‘passive’ pre-chamber technology to improve burn rates and reduce the likelihood of detonation. The passive design does not use a secondary injector and takes its combustion charge from the main chamber during the compression stroke. The passive system has demonstrated in race engine applications, a very high EGR (Exhaust Gas Recirculation) tolerance, so at engine operating points where Lambda 1 if the target AFR (Air-Fuel Ratio) setting, it allows operation at relatively high levels of EGR at high load. This helps to reduce the possibility of engine knock at heightened compression ratios. This allows the possibility to extending the compression ratio and benefit from the increased cycle efficiency.

 The active concept contains both a small spark plug and a low-flow direct-injecting fuel injector in a pre-chamber capsule. The active jet ignition is proposed by Mahle as the potential “ultra-high efficiency” application. The system operates by employing the secondary injector to maintain an easily ignitable charge inside the pre-chamber. This allows the system to exceed the established flammability limits in the main combustion chamber, reaching a Lambda value greater than 1.5. The main chamber is operated at this level which is actually close to diesel air-fuel ratios (AFRs), at which point, flame temperatures are sufficiently low to mitigate any NOx generation that might occur with richer operation.

Detailed view of pre-chamber assembly

Both active and passive versions of this pre-chamber technology are in general the same with regard to combustion principle: as both benefit from an outgoing turbulent radical jet, forced from the pre-chamber, which ignites the main chamber charge; this has the effect of amplifying the available ignition energy, thus delivering stronger and more reliable combustion in the main chamber - from multiple ignition sites and with fast burn rates.

It may seem a relatively simple system - but its complexity and attainment of required refinement should is a considerable task. However, advances in direct-injection gasoline systems’ flexibility and with careful consideration of pre-chamber geometry and nozzle selection - Mahle believes this technology is a key to significant powertrain efficiency gains - in terms of fuel consumption (easily rivaling diesel figures) together with reduced gaseous emissions.

Pre-chamber technology is not new; it has been used for decades in diesel engines (but now old technology - replaced by direct injection, common rail diesel technology) and Honda introduced it in the early 1970s for the first-generation Civic’s 4-cyl. spark-ignition gasoline engine, calling it CVCC (Compound Vortex Controlled Combustion).

Development testing has shown that NOx emissions were below 100 ppm (parts per million) at ultra-lean conditions are achievable in anticipation of Euro 7 targets. Used in conjunction with further technical improvements in friction, available coating technologies and charge management - a potential of 45% brake thermal efficiency has been demonstrated by Mahle. In summary - could this technology allow a gasoline engine to achieve the fuel consumption of a diesel? Mahle stated they have achieved this already - and that by combining downsizing, Jet ignition technology and a 48V hybrid system
could deliver even better results.

Friday, 29 January 2016

Technology Focus - Super capacitors for Automotive applications

I came across some very interesting new technology recently - specifically, the use of super-capacitors in automotive technology to solve a relatively simple problem - it is a good example of how innovative technology can find a place in day to day use, once the technology becomes cheap enough!

I encountered  vehicle start assistance devices that employ super-capacitors! but first of all - what is a super-capacitor! they are also known as ultra-capacitors but basically, they are the same as any other capacitor - an electrical energy storage device that stores this energy in a dielectric field between two electrical ‘plates’. In function, similar to a battery, the difference being that a battery uses electro chemical storage. The super-capacitor however has a much greater energy storage capacity when compared to traditional types of capacitor - this greater capacity has led to super-capacitors being employed in some fields where a traditional battery may have been used, but where they can provide a specific set of advantages over and above.

Battery and Super(or Ultra) capacitor comparison (www.maxwell.com)

Super-capacitors, like many other technical innovations, were developed for military applications and they have several benefits - they have a much lower sensitivity to temperature and can actually still perform very well at very low temperatures (-40 degrees Celsius). They also have excellent performance with respect to power flow density - this means they can accept repeated high capacity charging and discharging and this is a clear advantage when compared to a chemical battery (which requires a chemical reaction to take place during the energy conversion process, this takes a finite length of time thus increasing the response time to a sudden power demand). Another advantage is the life cycle - even under harsh charging and discharging duty cycles, super-capacitors maintain their performance and their expected life is much longer than a chemical battery for the same operating conditions - 10 years is expected as a minimum!

Super caps and batteries compared (www.koldban.com)

These devices are already in use in the automotive technology domain - in Formula 1 - the super-capacitor is ideal as a storage device for electrical energy from KERS and HERS energy recovery units. The high power flow density is ideal for this application as a replacement for the battery, or as a parallel device, in order to manage the complex energy flow and storage requirements during a race.

F1 KERS system layout that uses super-caps for energy storage (pre-2014 regulations system shown)

After some internet research I also found another interesting development for super-caps - vehicle flat battery assistance, also known as jump starting! I found some applications where super-caps have been employed for this - either as 'jump start' packs, or as units permanently installed on the vehicle alongside the chemical battery. In particular, commercial vehicles operating in extreme conditions at low temperature - Ice road truckers for example. In this domain they are ideal as the potential cost of downtime is very high for these vehicles. The additional weight and cost of a second storage device in addition to the battery is insignificant on a large truck or earthmover, especially when compared to the risk of not being able to start the vehicle at very low temperatures.

Super capacitor engine start module for permanent installation (www.maxwell.com)

Super-capacitors are ideal for mobile jump starting (as a jump start pack) as they require much less maintenance when compared to the use of a chemical battery. They are ideally suited for where a lot of energy is required in a short time i.e. to crank the engine, they can be charged quickly for use (seconds or minutes), hence the pack does not have to be maintained on charge when not in use. 

Mobile engine start assist pack employing super capacitors (www.koldban.com)

However, if the super-capacitors are combined in a jump start package, along with some clever electronics - it is actually possible to charge the super caps from any available low voltage source -  but how is this possible? well, the clever electronics is actually a DC-DC converter - which is a device capable of transferring electrical power at different voltages levels (input compared to output). These devices are already in use on certain vehicles, specifically those equipped with start-stop engine technology – the reason is that during  an engine start , on a vehicle equipped with a traditional style electric starter motor, the starting current draw is sufficient to cause a 'dip' in the vehicle system voltage. Some systems are sensitive to this and may cause the action of a start/stop system to be more obvious to the driver. To avoid this, these electrical systems are connected to a DC-DC power supply system, which maintains a constant voltage and power level to avoid any perceivable response to voltage fluctuation for driver evident systems like HVAC, entertainment and driver information.

DC-DC converter as part of the stop/start system components (www.bosch.com)

Super-capacitors have a higher energy flow density but not a high energy storage capacity. So, it is possible to charge the capacitors with a relatively small power source, the DC converter acts like an electrical transformer to step up the voltage to charge the capacitors with sufficient energy for a single start operation. This is where the capacitors win over a battery due to their power delivery capability. Another interesting development is that using this power transform capability - the capacitors could actually be charged from the dead battery itself - as long as it has some voltage and power capacity. This seems difficult to comprehend but you should remember that the normal failure mode of a chemical battery involves its ability to deliver high power for starting - it is less often the case that the battery cannot deliver any power, even a small amount over a longer period of time, so this fact can be utilised for charging the capacitors for a start assistance situation.

The question now is, what does a package as mentioned above look like? I have been lucky enough to get access to the very latest device that incorporates all the features above - it is sold in the UK by Sealey (www.sealey.co.uk) who have an exclusive licence to sell the device with the manufacturer. The device is sold as a 'battery less' jump start pack - it is small and light when compared to a battery based device. It needs no maintenance and charges from the dead battery, or from another vehicle, or via USB. It can also be used with the vehicle battery open circuit, for situations where the battery is completely dead!

Batteryless jump start pack (www.sealey.co.uk)

Sealey electrostart charging from the 'dead' battery (www.autoelex.co.uk)

In service, it works well, the package is aimed at passenger car users and can supply about 300 amps, this is more than enough power and the device also includes a diesel glow plug support mode to allow pre-heating time.

My personal view is that this technology will definitely been seen more and more in Automotive applications. Modern vehicles have complex energy flow requirements, and increasing electrification will mean that an electro chemical energy storage device alone, may not fulfil all the technical requirements. So, my opinion is, that to support all the energy storage requirements and consumers in forthcoming vehicle platforms, a balance of energy storage technologies will be required – including traditional style wet batteries, advanced batteries with new chemistries, capacitors and even mechanical storage (hydraulic, pneumatic, flywheel).

Saturday, 7 February 2015

Vehicle Wiring – Wiring Diagram Master class

Wiring diagrams! Not dissimilar to Marmite! Perhaps I should explain - you either love them, or you just don’t ‘get’ them. If you do have access to a wiring diagram, it can save you a lot of time when diagnosing faults or trying to understand how an electrical component or system works. So on the whole, they are useful, and they can save you a lot of heartache.

The biggest problem is consistency, or lack of it! Many manufacturers have their own idea of the best way to draw a vehicle wiring diagram, different representations that vary with manufacturer is the first of a series of confusing issues, add to this the increasing complexity of vehicles over time, plus the lack of harmonised wiring colours for vehicle electrical systems, and the whole thing becomes more complicated than it’s worth, especially when you can pay someone else to fix it!

In this blog feature, we’re going to show you the basic, most common types of wiring diagrams, and how to read them, so that you can decipher your auto-electrical hieroglyphics.

What is a wiring diagram?

Wiring diagrams are a graphical representation of the vehicles wiring system; they can cover the whole vehicle wiring system, or can just represent sub circuits or systems. With more modern vehicles, that have greater complexity, the wiring diagram is very often split into sections that represent the major vehicle circuit groups. There are different approaches and styles to wiring diagrams that vary according to the manufacturer. In addition, increasing complexity requires a different representation and visualisation than a simple circuit. In order to simplify this topic, lets take a quick look at the different ways of representing wiring systems:

Schematic Diagram:

This diagram type focuses on representing the actual flow of current around the circuit. Simple lines are used to show interconnections, and symbols are used to represent components. The drawing does not really represent to layout of the actual wiring system.

Pictorial Diagram:

Pictorial representation shows the circuit layout and position, using diagram elements that accurately represent the visual aspect of the components and circuit. They’re less useful for understanding how a circuit works, but, a pictorial view can help with understanding and explaining a circuit operation at a simple level

The above methods of visually representing electric circuits are often combined for vehicle wiring diagrams. Over the years different manufacturers have used various approaches and have often devised and developed something unique. It’s well worth familiarising yourself with the most common types your likely to encounter, then, you’ll be able to cope with most things you’re likely to encounter.

Some types of diagrams in use 

Some of the most commonly used approaches, by various manufacturers, for representing a vehicle wiring system are shown below. 

Current flow diagram

By far the most logical and useful type is the current flow diagram, used extensively by German manufacturers, and others (Japanese vehicles). Although the current flow diagram does not resemble the vehicle wiring layout in any way, shape of form, it uses current ‘tracks’ to represent sections of the wiring system, with cross reference markers where wires interconnect. Along the top of the diagram are the main supplies, the current flows down the page to the earth connection running along the bottom. The wiring system is broken into sub-sections, and with this approach it’s much easier to focus on a specific area (e.g. lights, ignition system) when following the circuit path. This type of diagram can be used to represent the more complex systems, such as those found on newer vehicles, in this case, the wiring diagram will be pages and pages of reading.

German Wiring

German based wiring colours are reliable and consistent, you’ll find the same basic colours used on German vehicles (VW group), as well as Fords and Vauxhalls from the 70’s and 80’s. Tracer colours are used to identify individual circuits that are generally fused. Here are the main base colours you’ll come across:
Main beam –White
Dip beam – Yellow
Side, panel and marker lights – Grey
Igniton circuits – Black
Battery feed – Red
Earth – Brown

Schematic Diagram

This diagram type shows the actual layout of the vehicle wiring harness, and the main groups of cable runs. Wires are shown as lines and are often identified with colour codes. The diagram type is more representative of how the wiring system is laid out in the vehicle, and specific, key points of interest, like earth points or main connector junctions are clearly identified as to their position on the vehicle, this can help loads when you’re trying to locate one of these. However, this type of diagram can be difficult to interpret, especially when you are trying to follow the path of an individual cable – my advice...photocopy the diagram and get your highlighter pen out! this will allow you to mark and follow the cable around the diagram. 

Classical wiring diagram

These were the first type of diagrams used to represent the vehicle electrical system, (which were, at the time, pretty simple). They are a combination of pictorial and schematic representation, and were sometimes in colour, showing the actual cable colours used. The diagram showed the actual layout of components, and each component was clearly identifiable as it was drawn as to closely represent the component itself. This meant less reliance on a diagram ‘key’ when using the diagram. This type of diagram works for less complicated electrical systems, but later vehicles (80’s onwards) were a bit too complex for the system to be represented with a diagram of this type.

British Wiring

Most British classics from the 60’s, 70’s and 80’s used BSAU7 wiring colours; this consisted of basic colours for the main function groups, with tracer colours to identify sub-circuits. Later vehicles were appended with additional extra wire colours to cover the additional complexity from extra accessories that were being fitted, like electrical windows, central locking, engine management etc. The basic colours you’ll see are:
Headlights – Blue
Sidelights – Red
Battery Feed – Brown
Fused battery feed – Purple
Ignition feed – White
Fused ignition feed – Green
Earth - Black

Other types

The above diagrams will cover the majority vehicles encountered. However, there are always exceptions to the rule – for example, French, or eastern block cars. This is where the ‘diagnostic skills without diagram’ come to the fore. However, if you encounter a more obscure vehicle, complete with a wiring diagram, then that’s still a good position to be in. Just be aware of the idiosyncrasies that you may come across, for example, French cars tend to use a simple wiring colour scheme with about 4 basic colours, with the wires ‘tagged’ at each end. The problem with this is that half way along a wiring loom; you won’t be able to tell one wire from the other! Note also that the wiring diagram for these vehicles is just as confusing! with many variations and undocumented changes, irrespective of this, something is always better than nothing, and a diagram will always help.


In summary, possessing the correct wiring diagram is a great thing. It is well worth studying the diagram, to familiarise yourself with your vehicles peculiarities and details, even when don’t have any electrical problems. Then, when electrical gremlins hit (and they inevitably do), or, you want to do some mods or upgrades, then you’ve done your homework in advance – forewarned is forearmed! There are some general pointers below that offer ‘best practice’ advice for dealing with extracting and processing the info. you need from your diagram:

  • Photocopy and mark-up – Take your original diagram, store it carefully and keep it in good condition. When you need to use it, photocopy it. You can even photocopy and enlarge the specific section you need to help see the detail. This allows you to keep the original pristine, so that you can mark up or use highlighters when you need to, to aid understanding and use of the diagram
  • Analyse in sections – Don’t try to use the whole diagram at once, break it down into sections (mentioned above). If you’re using the diagram to help diagnose a fault, break the fault down into sections as well. This is a more methodical approach and will help you deal with complex problems
  • Identify cable colours and connection points – Study the diagram for the circuit you are looking at. Try to separate it out and identify the basic colours used for feeds, earths, switches etc. This will help you when you are actively tracing cables on the vehicle

Final Advice

You are always going to come across a situation at some point, where you don’t have a wiring diagram. So, it is also worthwhile when you are using one, trying to make a mental note of how the diagram relates to the vehicle system itself. Also, any general conventions that you may come across – for example, the way headlight circuits are generally fused.
It’s well worth getting into the practice of being able to cope without a diagram and this will help you be more self-sufficient so that you can diagnose faults or understand a cars electricky, in the cases when you don’t have one.

Wednesday, 14 January 2015

Technology focus - Steel pistons

In the continuing battle for combustion engine CO2 reduction. Engine manufacturers are hunting for new technologies that can contribute in some way – however small. An emerging trend is looking closely at loss reduction - and in particular, reduction of engine friction. One specific technology under investigation and being adopted, is the use of new materials for piston manufacture – in particular, the use of steel as opposed to aluminium.

Let’s review the current situation – in a typical, current passenger car diesel engine, loads and temperatures are high, the safe temperature limit for an aluminium piston is around 400 degrees centigrade. With these modern engines, these material limits are already being approached – failure is normally associated with cracks forming at the combustion chamber bowl rim - so how can steel help? Steel is heavier with lower thermal conductivity! Well, heavy-duty truck engine manufacturers have been using steel pistons for a while – steel is much stronger than aluminium, so with an advanced design, a steel piston assembly can actually be lighter than an equivalent aluminium piston. Thus it is possible to compensate (nearly fully) for the weight disadvantage of steel. This benefit also brings the advantage of additional strength – protecting for peak pressures that will become even higher in future. This increased strength, in combination with the engine design, can be utilised to reduce the deck height of the engine, thus reducing overall height, which has a packaging benefit.

Cutaway of a steel piston design (KSPG)

With respect to the engine cycle, the lower heat conductivity can actually be an advantage as cycle temperature is increased, which has a thermodynamic benefit. Higher combustion chamber temperatures can be reached than with aluminium piston engines so that ignition quality increases, while the combustion duration is reduced. The result is lower fuel consumption and pollutant emissions. The biggest benefit however comes in the form of reduced friction – a steel piston only expands about a quarter of the extent of its aluminium equivalent. When fitted into an aluminium cylinder block, the aluminium housing expands more than the steel piston – and the result is greater tolerance of the piston within the cylinder, with correspondingly less friction - as the piston/cylinder assembly alone causes between 40 and 50 percent of the mechanical friction - the potential for efficiency increase is significant in the lower and middle speed ranges (important in real world driving conditions where useful consumption benefits can be achieved). In addition, the lower thermal expansion of the steel piston, compared with aluminium, also means that the designers are able to reduce the working tolerance between the cylinder wall and the piston, this reduces pollutants and untreated emissions.

Steel and aluminium piston designs - steel piston on the left is much smaller

When used in a cast-iron cylinder block (Diesel engine), the steel piston enables reduced working tolerance when the engine is cold (lower heat expansion and the resulting potential of having a significantly tighter clearance between the piston skirt and the cylinder bore), with an appropriate tolerance being maintained when the engine is warm (due to complimentary expansion of the piston and block material) – the reduced clearance at cold conditions leads to less noise at cold starting, as determined by the piston contact changeover at the crankshaft angle of top dead centre.

BSFC plot showing improvements gained by using steel pistons compared to aluminium

So in summary, steel pistons have a clear advantage – and when used in conjunction with other technologies (surface treatments for piston skirt and bore) considerable benefits in fuel consumption, CO2 and efficiency can be gained. In testing engine manufacturers reported the following results:

Power increase of around 2.5 % with the same calibration
Nearly 2% improvement in torque/fuel consumption at a fixed, medium engine speed
Reduced fuel consumption by up to 4% for a given NOx emission
Heat exchange reduced by 1%, energy transferred to cylinder work

Steel piston design as used by Daimler

Steel pistons are just one technology being explored, there are many others so keep an eye on this blog for more technical information on the latest developments in automotive engines and powertrains

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).

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: