You may have heard the term – Engine Downsizing. It’s a hot topic in the automotive world and many car manufacturers are hurriedly developing ‘downsized’ engines to meet current and future emission regulations. Some, like VW are already ahead of the game and have these engines in production to buy today. But what does this concept actually mean? What are the benefits to this approach? And what are the technical challenges?
Improved fuel economy and reduced CO2 emissions are the major challenge faced by vehicle manufacturers developing future passenger car powertrains. Gasoline engine downsizing is the process whereby the speed / load operating point is moved to a more efficient operating region (at higher load) through the reduction of engine capacity, whilst maintaining the full load performance via pressure charging. Downsizing concepts based on turbocharged, direct injection engines are a very cost effective solution. The most significant technical challenges for such fuel efficient turbocharged GDI (Gasoline Direct Injection) are providing the required low end torque and in addition, a suitable transient response to give the required levels of engine flexibility and drivability.
Fig 1 - What is downsizing?
In downsized engines, by applying a refined single stage charging concept, the full engine torque can be available as low as 1250 rpm. This can be combined with a specific power of 80kW/l. With the use of dual stage boosting, High torque, with a specific power > 140 kW/l is achievable. In conjunction with some other new technologies - exhaust cooling, cooled external EGR at high load - this results in a significant improvement of the real world fuel economy. In addition, efficient spray guided, stratified charge systems are utilised to gain further improvements. The overall goal is to create an engine with excellent high load performance and durability, and to operate the engine in this high-load region as much of the time as possible. The combination of GDI and turbo charging, implemented on a small displacement engine, is the good basis to combine high real world fuel economy with an acceptable performance - even under a stringent CO2-scenario.
Fig 2 - Downsized engine - torque/speed/fuel consumption - development over time
In the past gasoline engines were perceived as a very cost effective Powertrain solution. Emissions were not that important as long as three way catalyst technology was used to ‘mop up’ the exhaust. Fuel economy was not really the primary target. If you wanted good fuel economy, you’d buy a diesel! But, on the other hand, diesel engines needed considerable technological effort in order to meet emission legislation, and diesel engine developers were allowed to introduce rather costly technologies in order to meet these emission targets. The key word was: “emission is a must; low fuel consumption is nice to have”. In current times, there’s a new direction, and that is CO2 reduction. This discussion is significantly enhanced by a penalty tax for OEMs (Original Equipment Manufacturers) not meeting future CO2 limits. Now as CO2 is seen as a harmful emission, gasoline engine developers get a chance to invest in some fuel economy technology. With Gasoline engines in particular, downsizing/downspeeding concepts based on turbocharged GDI, seem to be in pole position for the race of the most accepted technology for reducing fuel consumption - whilst keeping the additional benefit of performance and drivability (when compared to a traditional diesel).
Fig 3 - The technical challenges to achieving a downsized engine concept can be considerable
Technology and Challenges of Downsizing
So what are the technical aspects of a downsized engine? It’s a small engine that produces high power. So, it’s operating much closer to the thermal and physical limits of the materials used in construction of the major engine components. It needs to have the following attributes:
- A well designed combustion system that allows high compression ratios to promote efficiency
- It needs to have excellent low speed torque, as most of the engine power is produced via the torque – like a diesel engine
- It needs to have good, transient response to give an appealing performance to fulfil driver expectations
- Good fuel economy – reduced requirements for full load enrichment
- Very robust and durable base engine design
In order to achieve the above engine profile, there are a number of technologies in development and use. They can be used in combination with each other, or in conjunction with other technologies for CO2 reduction (like mild-hybridisation and start/stop) - some of the technologies specifically involved in a 'downsized' engine package are:
- Direct Gasoline Injection
- Turbo and super charging
- Cooled EGR
- Active Exhaust cooling
- Variable valve timing
Fig 4 - Cooled turbocharger housing - no need to run rich to control high exhaust temperatures, thus saving fuel (Source: AVL)
This is another similar approach, often mentioned in the same context as downsizing. It involves moving the most frequently used engine operating point, to where it is more efficient (as downsizing does, but in this case, lower speed instead of higher load). At lower engine speed, a higher torque is needed to maintain the required power. The advantage of low speed operation is that friction losses are reduced (due to lower rubbing speeds between components). In addition, this concept provides real fuel savings as fuel consumption efficiency often increases with lower engine speed. The technical challenge is that high torque means high load on all engine components, this increases material costs to cope with these loads. Also, Down speed engines need to have a fast torque build up, in order to meet the requirements for transient response. This requires as a minimum pressure charging, and in addition, perhaps some other technical approach to be able to produce the required torque – for example, electrical assisted Powertrain, or electrical assist for the turbo/supercharger.
Another less common but viable alternative to downsizing is de-rating, especially for diesel engines. De-rating means to limit the power output of a given engine design - that is, not going to the specific power extremes of the design (power density typically limited at ~ 45 kW/L). The advantage here is that this lessens the requirement for a sophisticated engine design with expensive high-end components, due to lower PFP (peak firing pressure). Of course, for such de-rating concepts, viability has to be investigated with respect to the expected production volume, costs, image, regional market aspects, etc. - these factors all have to be taken into consideration.
Fig 5 - Comparison of concepts downsizing vs. de-rating (Source: AVL)
De-rating also offers the potential of commonality between Gasoline & Diesel engine family production. With increased number of common parts with gasoline engines, this leads to increased production volumes and consequently lower cost.
It’s a fact that downsizing is the current way forward; you can see in the market that most manufacturers have, or are currently developing, engines with lower displacements and better CO2 figures - maintaining the same power density. That’s all fine – if we can squeeze more out of an engine, increase its efficiency whilst maintaining durability, then that’s a win-win all round.
Developments in material technology and engine design have facilitated this opportunity, but future powertrains will need more than just smaller displacements to achieve forthcoming emission regulations, without sacrificing the driving experience.
So, downsizing and down speeding will be adopted in conjunction with other technologies. The reason is the smaller engines produce less torque, even highly boosted smaller engines, and the market will not accept ‘sluggish’ vehicles in today’s modern traffic. As we move towards micro and mild hybrids, and start/stop technology, the electric motor, as a torque supporting element becomes even more viable! An electric motor can produces full torque at low speed, and for short time torque boosting, is an ideal option to fill the gap in future downsized powertrains.