**9. Conclusion**

The 2-Stroke cycle combined with Compression Ignition is a promising solution for high speed engines, particularly for small passenger cars and for light aircraft (power < 140 kW). In comparison to a corresponding 4-Stroke engine, the double cycle frequency yields the fol‐ lowing advantages: higher power density, with ensuing possibility of downsizing and/or down-speeding the engine; higher mechanical efficiency, in particular with the piston-con‐ trolled ports (no valve-train); lower soot and NOx emissions at partial load, thanks to the higher air excess; possibility of having a high content of residuals within the cylinder with‐ out an external EGR system.

craft application requires strong modifications from the design used for passenger car en‐ gines, so that the transformation of an off-the-shelf Diesel engine has a cost very close to a

Advances in The Design of Two-Stroke, High Speed, Compression Ignition Engines

http://dx.doi.org/10.5772/54204

179

At the moment of writing this document, the field of High Speed Compression Ignition 2- Stroke engines is an open research area. A lot of ground still has to be covered in order to develop reliable prototypes, able to practically demonstrate the theoretical advantages found by means of CFD simulation. In the automotive field, the concept may also find an application to the so-called "range-extenders", i.e. internal combustion engines designed to recharge the batteries of electric vehicles. Here, the compactness and the low pollutant emis‐ sions level of the 2-Stroke cycle could play a fundamental role. In the aircraft field, the effort will be focused for keeping the engine design simple and reliable, in order to be competitive

with the 4-Stroke SI engines also in terms of production and installation cost.

brand new project.

**Nomenclature**

AFR Air-Fuel Ratio

DI Direct Injection

DR Delivery Ratio

1D/3D One/Three-Dimensional

Ap Area of the piston (cross section)

BMEP Brake Mean Effective Pressure

CFD Computational Fluid-Dynamics

EGR Exhaust Gas Recirculation

BSFC Brake Specific Fuel Consumption

EPO/EPC Exhaust Port Opening/Closing

FMEP Friction Mean Effective Pressure

IMEP Indicated Mean Effective Pressure

HSDI High Speed, Direct Injection

IDI In-Direct Injection

Rpm Revolutions per minute

EVO/EVC Exhaust Valve Opening/Closing

BDC Bottom Dead Center

Aeff,av Effective average area of transfer and exhaust ports

Since 1990, many prototypes have been designed and built, according to quite different con‐ cepts. The two most interesting designs, in the authors' opinion, are the Uniflow scavenging, with exhaust poppet valves, direct injection with bowl in the piston, and Loop scavenging, with piston controlled ports, direct injection and bowl in the cylinder head. Both solutions adopt an external supercharging system, so that lubrication can be the same of a convention‐ al 4-Stroke engine.

The uniflow design is closer to the 4-Stroke engine, and its bigger advantage is to share most of the components with mass production engines. In particular, the combustion system and the valve-train is the same of passenger car Diesels. Conversely, the loop design requires a much bigger effort, since the combustion system must be developed according to new con‐ cepts, and a number of minor issues concerning piston rings and liner durability must be carefully addressed. The reward for properly addressing these issues is a very compact de‐ sign and an excellent mechanical efficiency.

As far as scavenging is concerned, it is a widespread opinion that Uniflow is always better than Loop, in terms of efficiency. This is not the final outcome of the authors' investigations, whereas it was found that a strong support by CFD simulation can help the designer to close the gap between the two designs and even get a higher quality of the gas exchange process. The same CFD support is the key to develop efficient combustion systems, without need of a swirl motion within the cylinder.

In this document, the development of two different 2-Stroke High Speed Direct Injected Die‐ sel engines is presented for two applications: small passenger cars (1.05 liter of capacity, 3 cylinder, power target 80 kW@4000 rpm) and light aircraft (1.8 liter of capacity, 3-cylinder, power target 110 kW@2600 rpm). In both cases, the design guidelines are discussed.

The superiority of the 2-S design in comparison to the 4-S stroke cycle is demonstrated by means of CFD analyses, performed by means of experimentally calibrated models. In partic‐ ular, the passenger car 2-S engine is able to provide, from 2250 to 4000 rpm, a brake power higher than the peak value of the reference 4-Stroke engine (1.25 liter, 4 cylinder, turbo‐ charged, peak power 65 kW@4000 rpm). Furthermore, engine-out soot emissions at partial load are about one order of magnitude lower, while NOx can be controlled without an exter‐ nal EGR system. As far as the aircraft engine is concerned, the 2-S design yields a big weight saving in comparison to a 4-Stroke engine delivering the same power; furthermore, the air‐ craft application requires strong modifications from the design used for passenger car en‐ gines, so that the transformation of an off-the-shelf Diesel engine has a cost very close to a brand new project.

At the moment of writing this document, the field of High Speed Compression Ignition 2- Stroke engines is an open research area. A lot of ground still has to be covered in order to develop reliable prototypes, able to practically demonstrate the theoretical advantages found by means of CFD simulation. In the automotive field, the concept may also find an application to the so-called "range-extenders", i.e. internal combustion engines designed to recharge the batteries of electric vehicles. Here, the compactness and the low pollutant emis‐ sions level of the 2-Stroke cycle could play a fundamental role. In the aircraft field, the effort will be focused for keeping the engine design simple and reliable, in order to be competitive with the 4-Stroke SI engines also in terms of production and installation cost.
