Technology focus - Engine Downsizing and Downspeeding

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

Why Downsize?

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)

Downspeeding

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.

De-rating

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.

Summary

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.

Engine Combustion - Spark Ignition (Gasoline)

Engine combustion is a fascinating topic to gain an understanding of, particularly when comparing compression and spark ignition, the fuels used and their properties needed for each respective type. Looking at details, and the current trends in technology. It’s not hard to see the convergence and similarity between gasoline and diesel engine fuel systems and combustion. Let's look in detail first at spark ignition based combustion

Spark ignition

For spark ignition combustion, the mixture is prepared completely prior to combustion (outside of the combustion chamber) - that is, the fuel is introduced to the air - fully atomised - and in theory, this mixture is uniform in its distribution. So called ‘homogeneous’, the amount of fuel in proportion to air should be chemically correct. What this means is that there is enough air, containing oxygen, to fully oxidise or ‘burn’ all of the fuels volatile content. Sounds quite straight forward, but in practice, not that easy considering all the operating conditions that a vehicle engine has to encounter. This mixture is compressed in the cylinder as the cylinder volume decreases due to the piston rising towards top dead centre (TDC), the pressure increases, with reference to simple gas laws, the temperature of this mixture also increases, but not sufficiently to reach the ignition point of the fuel/air mixture.

So far then we have mixed fuel and air and compressed it. But there are many hurdles for the engine designer to overcome to be able to do this efficiently for a multi-cylinder engine. These days, fuel is injected into the air stream, near the inlet port, but remember the carburettor (or even a single point injector). The mixture preparation occurs away from the point of entry into the cylinder, thus, distributing the mixture evenly to each cylinder, with the same amount of fuel for each cylinder, for a given operating condition, was a real headache for the engine designer.

Fig 1 - Single a) and Multi-point b) injection system layout 

Why? - well, in order to get the best possible performance and efficiency out of an engine, the individual cylinder contributions must be as even as possible, with as little variation as possible. Even small variations have a dramatic effect on the overall engine performance, so even mixture distribution is key to this, and impossible to achieve fully with a centralised mixture preparation system like a single carburettor shared between cylinders.. In addition, the distance that the mixture travels in order to get to the cylinder has another effect, that is the possibility that the fuel and air may separate during transit - the fuel literally drops out of the moving air becoming liquid droplets again (instead of a finely atomised spray). This is known as wall wetting, and causes flat spots due to instantaneous weak mixtures being introduced to the cylinder, this effect is much worse at low temperature (hence the need for the choke in days gone by, to enrich the mixture when cold) and during transient operation, where the air accelerates faster than the fuel (hence the need for an accelerator pump, to richen the mixture during accelerations). These were some of the arguments for the move to port fuel injection to each cylinder, thus contributing to improving efficiency and reducing emissions.

Back to combustion - fuel and air is mixed and compressed, now we are ready to produce some work. In a gasoline engine, I am  sure we all know that an electrical spark or arc is used to start combustion. We mentioned before that the mixture temperature is raised, but not beyond its ignition point. The intense electrical arc produced by the spark plug at it’s electrodes creates a localised heating of the mixture, sufficient for the fuel elements to begin oxidising and combustion of the mixture starts with a concentric flame front growing outwards from the initial ignition kernel. Once this process is initiated, it perpetuates itself, there is more or less no control over it. We just have to hope that the mixture is prepared correctly to sustain this flame so that is consumes all of the mixture, burning it cleanly and completely. The technical term for this type of combustion is ‘pre-mixed’. The engine is ‘throttled’ to control the mass of mixture in the cylinder, and hence its power output. The throttle being a characteristic of the gasoline engine.

Fig 2 - Flame propagation in a gasoline engine via optical imaging system (source: AVL)

An important point to note is that the speed that this flame travels across the combustion chamber is important, the typical  flame speed, travelling through an air/fuel mixture, would be far too slow in a combustion engine (approximately 0.5 metres per second). So, we have to speed things up. The way this is done is via cylinder charge motion, or turbulence. The turbulence is generated via induction and compression processes in conjunction with the combustion chamber design and has the effect a breaking up the flame front, increasing its surface area, thus increasing the surface area of fuel mixture for oxidation. Assuming a normal combustion event, the flame front grows out to the periphery of the cylinder where it decays once all the mixture is burned.

Fig 3 - Charge motion speeds up the combustion process, in a gasoline engine this is generally known as 'tumble'

Of course the timing of this event is essential! Ideally, we want the cylinder pressure, forcing down on the piston, to occur at the correct time relative to the crank angle. Seems obvious - too soon and we may be trying to push against the rising piston, too late and the piston is already moving down the bore, hence the expansion of the gas won’t do any work and the energy will be wasted as excessive heat in the exhaust.. A simple analogy would be to imagine pushing someone on a swing - too soon and the effect is collision, too late and the effect is no force transmitted - well, it’s the same in the engine cylinder. What engine engineers do know, is that half of the total fuel energy should be released at around 8 to 10 degrees after top dead centre. This can be measured by cylinder pressure analysis during an engine test. Hence, with a new engine design, the appropriate ignition timing can be mapped via monitoring the cylinder pressure for energy release, as well as knock, in order to map the correct value for a given engine operating condition. In summary then, the key points to consider regarding the spark ignition engine:

• The fuel/air mixture is prepared externally, and ignited via a timed spark
• The engine power is controlled via throttling, which reduces efficiency, particularly at part-load
• The compression ratio is limited by self-ignition of the fuel/air mixture
• In operation, engine maximum torque is limited by abnormal combustion (knocking)
• Cylinder to cylinder variation (due to fuel distribution problems, and other factors) reduces the efficiency of the engine and is significant in a spark ignition engine