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.