Direct-EGR tillämpat på HCCI

Sammanfattning

The Homogeneous Charge Compression Ignition (HCCI) engine is promising in terms of low

NO

x, low particulates and high efficiency. Due to its homogeneous charge, where the

combustion starts almost simultaneously from a number of points in the combustion chamber,

only lean mixtures are possible to burn because of the resulting rapid combustion, which is much

faster than the combustion in an SI-engine or a diesel engine. When richer mixtures are used the

combustion will start earlier and also be more violent.

To phase the combustion later and lower the combustion speed, external cooled EGR can be

used. The EGR-system used in this thesis uses a divided exhaust channel where one channel and

its corresponding valve are used only for transporting EGR back to the intake system.

Consequently the EGR-valve is connected to the intake pressure. This system is called Direct-

EGR (D-EGR).

Engine tests verified that the CA50 indeed is phased later when cooled EGR is used. The more

cooled EGR that is available in the combustion chamber, the later the CA50 is phased. Hence

power output of the HCCI-engine can be improved quite a bit when large EGR-rates are used,

because of the possibility of increasing the injected fuel amount.

Simulations in GT-Power were made to determine how the EGR-valve lift and exhaust valve lift

should be made to transfer a certain amount of EGR with as good total efficiency as possible.

The simulations showed that two different valve timing strategies should be used. Which one of

the two strategies that should be used depends on the pressure difference between the intake and

the exhaust as well as the engine speed. These valve timing strategies are called the Major lift

strategy and the Minor lift strategy.

To determine the efficiency of the D-EGR-system, a comparison with an ordinary external EGR

system was carried out. The comparison showed that the D-EGR-system performs well,

especially at high load and low engine speeds. At high engine speeds the increase in flow losses

affects the performance in a negative way for the D-EGR-system. At low load the intake

pressure may exceed the exhaust pressure, which makes it unbeneficial to do a part of the

exhaust stroke with the EGR-valve open towards the higher intake pressure.

In terms of transient response the D-EGR-system was proven to be fast, with a possibility to

change the EGR-rate in just a couple of cycles.