HCCI-Combustion in the Z Engine - Part II 2012-01-1573

The most common car engine is a 4-cylinder 4-stroke engine. The car manufacturers have a great pressure to lower the cost of the cars and this deal also with the engines. The challenges are the coming new emission norms (for example EURO-6) and also the customer acceptance, because of the fact that the car drives are used to the 4-cylinder engine and they want to have the same driving fun also from the new engines. A 2-cylinder 2-stroke engine has the same power output and torque as a 4-cylinder 4-stroke engine and thus it offers the same driving fun. Equal balancing is easy to make without some big additional costs, if the gas exchange of the engine is made by using poppet valves and camshafts. As there are only about 70% of the moving parts in the engine, its acceleration is even better than by a 4-cylinder engine.

One of the latest developments in 2-stroke engines is the Z-engine, having the compression partially transferred outside of the working cylinders. This offers new thermo dynamical possibilities to adjust the working cycle and the combustion. As there are methods to control the temperature at TDC, a HCCI combustion is possible in the Z-engine at all loads. This lowers significantly the cost of the engine, as no urea injection, or NOx catalyst is needed to pass the coming EU-6 emission norm. The cost of the Z-engine is lower also because of the fact that it has only 2 working cylinders instead of 4.

In 1999, Aumet Oy began to research this 2-stroke car diesel engine, called the Z-engine, in co-operation with the Internal Combustion Engine Laboratory at the Helsinki University of Technology (HUT) and the Energy Technology Department at the Lappeenranta University of Technology (LUT). So far, four master's theses, two SAE Papers and four Fisita Papers have been completed on the subject. Modern simulation tools, such as Star CD, GT-Power, Diesel RK and Chemkin have been used. The prototype engine made its first start in December 2004 and testing of the engine has been made two years in a testbench.

In the HCCI combustion simulation of the Z-engine, a 4-dimensional ignition delay map, calculated with Chemkin and integrated in Diesel RK, has been used. The simulations and tests with the test engine show that the Z-engine has a very good efficiency, especially at part load. A HCCI combustion at all loads is possible in the Z-engine, with lambda about 1,8-2,3 and EGR-rate 10-40%, depending of the load. The TDC temperature at part load is about 800 K and at full load (BMEP 27 bar) about 700 K. The HCCI ignition, triggered with a pre chamber spark plug or small amount of fuel injection, occurs between 0°-20° ATDC and this limits the pressure and maximal temperature. No knock is present, as the ignition occurs always at the right side of the NTC (negative temperature coefficient) regime. NOx values are very low as the maximal temperature at full load is about 1900 K, because of the low starting temperature of the combustion, intern EGR and the expansion during the combustion. Intern EGR and active radicals stabilize the combustion and lower the activation energy needed for the ignition.