Technology Modules for Future Combustion Engines

A key role in the efforts to achieve a climateneutral powertrain world is being played by a good old friend: the combustion engine, which is the dominant type of powertrain on the roads today. Despite growing market shares for alternative powertrains, the absolute number of combustion engines being produced is currently on the increase. Furthermore, synthetic fuels are an option for climateneutral long-distance mobility. As a result, IAV sees further development of the combustion engine as one of the central tasks in powertrain engineering. Improving efficiency is just as important as achieving zero-impact emissions.

Spark-ignition engines currently on the market already achieve peak efficiency of roughly 38 percent, with diesel engines in passenger cars even topping that with a maximum of up to 46 percent. However, these best-map values apply to a lesser extent in everyday vehicle usage by customers, where current engines achieve values of 25 percent (spark ignition) to 29 percent (diesel). To achieve mean operational efficiency of 40 percent under real-world operating conditions, future spark-ignition engines will have to achieve peak efficiency of 45 to 50 percent. Target peak efficiency for diesel engines is another two to three percent more. IAV is currently working on the corresponding technology modules.

Today already, spark-ignition engines stand out with extremely low emissions due to their stoichiometric operation in conjunction with a comparatively simple exhaust gas aftertreatment system once the system has reached operating temperature. However, before the exhaust aftertreatment system reaches operating temperature, pollutant emissions are discharged untreated into the environment. This is the case with a cold start or restart in a hybrid powertrain.

One particularly interesting option here consists in preconditioning the fuel, for example with heated injection nozzles, as well as thermal encapsulation of the complete combustion engine. The latter option offers the possibility of keeping residual heat from previous journey sections in the engine for as long as possible.

Heat loss through the cylinder walls must be reduced as far as possible in the interest of efficient conversion of the energy contained in the fuel. But at the same time, when operating at high load, the heat needs to be dissipated quickly to prevent self-ignition in the mixture which could completely damage the engine. Phase-change cooling is one way of solving this trade-off. These systems have far greater cooling capacity, as a phase change lets the coolant absorb additional energy when faced with high cooling demands. Similarly, an add on feeds exhaust heat to the system which is harnessed in a piston steam expander. Initial tests at IAV indicate potential consumption savings in the WLTP of around 15 percent.

Efficiency can be improved above all by reducing the self-ignition tendency. The associated possibility for increasing geometric compression opens up further clear potential across the entire map. IAV has been working for a while now on a combustion process with injection pressures exceeding 1000 bar. In conjunction with an adapted injection strategy, this approach shows clear potential. Pre-chamber ignition is a method of reducing the knock tendency to achieve a distinct improvement in efficiency that has proven its worth in motor racing. Implementing an active pre-chamber ignition (i.e. active feeding of fuel into the pre-chamber) brings the additional advantage of high air dilution in the mixture without jeopardizing the stability of the combustion process.

Diesel engines are already highly efficient per se. But future acceptance of these powertrains depends crucially on bringing nitrogen oxide levels down to “zero impact” levels. The challenge with current exhaust gas aftertreatment technologies consists in achieving very low limit values under all real driving conditions rather than minimum emissions in typical standard driving cycles.

This is difficult above all when the engine is operating at very high and very low load. Particularly in very small engines in relatively large and heavy vehicles, very high load triggers a drastic increase in emissions if the space velocity in the catalyst is too high and the exhaust gas recirculation rates are too low. Basically, the only remedy here consists in appropriate dimensioning of the engine and the exhaust treatment system as well as in tandem SCR systems with two AdBlue metering points.

Immediately after a cold start and when operating at low engine load, it is above all the low exhaust gas temperature that gives developers cause for concern. The exhaust treatment system is still too cold or cools down as a result of low-load operation and no longer achieves the desired conversion rates. A key technology for reducing NOx emissions under these conditions is active cylinder temperature management for good EGR compatibility and the use of throttle-free charge control. When this is inadequate, as is the case immediately after a cold start, one effective method consists in increasing the exhaust temperature, for instance by installing a heated catalyst.

There is also the possibility of using active NOx adsorption catalysts or passive NOx adsorbers to buffer untreated nitrogen oxide emissions generated after the engine starts up. The energy contained in exhaust gas sinks drastically when using multistage turbocharging systems so that single-stage turbocharging with an electrical booster compressor is an attractivealternative.

The technology modules named above are just a few examples of the available options which can be combined appropriately for more efficient “zero impact” engines. Further developments of both diesel and spark-ignition combustion engines must also consider the use of climate-neutral synthetic fuels.

In order to get there, once these climate-neutral synthetic fuels are available they could act as a “drop-in” solution leading to a marked CO2 reduction in the new and existing fleet. However, first of all the CO2-reducing effect has to be acknowledged by law at short notice.

The article was published in automotion 02/2019, the automotive engineering magazine of IAV. Here you can order the autmotion free of charge.