Four-Stroke Piston Engines: Potential and Trends

The required drastic reduction in fuel consumption and, consequently, in carbon dioxide and pollutant emissions will decide the future of piston engines. This aim requires the systematic analysis, improvement, and control of energy conversion from the chemical energy of the fuel to heat and, further, to work. Combustion plays a central role in this process.

The distribution of fuel droplets within the air in the combustion chamber, their fast vaporization, and turbulence during mixture formation in every elementary volume of the combustion chamber are decisive for the efficiency of energy conversion, for local and momentary temperature and pressure values, and for the combustion products from CO2, H2O, and N2 (complete combustion) to NO, NO2, OH, H, and O.

A major problem during the optimization of a combustion process is conservation of obtained values of configurations for every combination of load and speed, for variable ambient conditions in terms of pressure, temperature, and humidity, and for transient operating conditions.

The control and optimization of combustion requires the actuation and regulation of the supplier’s functions independent of the crankshaft rotational speed. The thermodynamic process sequences around the combustion itself, so that scavenging, mixture formation, and heat transfer become modular.

There are one or more stages in supercharging/turbocharging, tuning of the pressure waves within intake and exhaust ducts, fully variable value control, and internal mixture formation by direct fuel injection, controlled self-ignition, exhaust gas recirculation, management of heat transfer, and increasing the effective compression pressure of the air-fuel mixture. Further reduction in friction losses and engine weight and the further development of exhaust gas treatment are measures for process improvement.

The adaptation of all process stages to give combustion that is as efficient as possible would enlarge the torque/speed with minimum brake-specific fuel consumption (bsfc) and pollutant emission. Constant values at minimum levels for all torque and speed combinations are not achievable in practice.

A lower power at high effective energy density, and thus at higher thermal efficiency, is given by reducing the swept volume (“downsizing”). One solution is to cut the heat input in a number of cylinders of a multicylinder engine by closing their intake and exhaust values, with concomitant interruption of fuel supply. A second solution is to keep the swept volume at the downsized value, but with displaceable maximum effective energy density. This is possible by intensive means (related to the specific work of the cycle) and by extensive means.

Increase in the Effective Energy Density

Intensive ways to increase the energy density of an engine are related in general to the path of the thermodynamic cycle. This is not applicable for a downsized engine, which has the full load at low power. Therefore, in this case, the energy density can be increased only in extensive mode. This means an increase in heat input and, thus, more fuel and more air. With the given swept volume, this is feasible with turbo- or supercharging. For currently developed piston engines, turbo- and supercharging is generally the basis of every downsizing concept.

In this mode, the torque becomes 2.5 times higher when operated with both charging systems than for a basically aspirated compact engine. The charging in a large torque/speed region is, however, linked to the volumetric efficiency of the engine.

However, this measure is not sufficient if extreme limitation of pollutant emission is required. In this case, the fuel must be eliminated from scavenging, being injected into the cylinder only after the scavenging. On the other hand, direct fuel injection can allow mixture stratification within the combustion chamber and, consequently, to controlled self-ignition at partial load by well-defined burned gas zones.

The main advantages of controlled self-ignition are an increase in thermal efficiency and a noticeable decrease in pollutant emission. Furthermore, direct fuel injection allows a perceptible increase in the compression ratio in SI engines under the knock limit, with similar advantages regarding thermal efficiency and pollutant emission.

Last word

However, elevated energy density obtained by engine downsizing generally sharpens the problem of heat loss by cooling and exhaust gas. The management of heat transfer from the cylinder by measures within the cooling circuit is an important development target for future piston engines. Regarding exhaust heat, the general utilization of downstream turbines seems to be a necessity.

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