We have always heard about two types of engines which
dominate the automotive market when it comes to internal combustion engines,
namely: Spark Ignition (S.I) and Compression Ignition (C.I). Homogeneous Charge
Compression Ignition (HCCI) engine is a blend of both S.I and C.I engines.
We all know that air-fuel mixture in a S.I engine is a
homogeneous charge (i.e.) air and gasoline easily mix together at room
temperature and require a spark plug to be ignited. Diesel on the other hand is
less volatile and C.I engine can combust fuel as soon as it reaches the auto
ignition temperature without the help of spark plugs. HCCI engines burn
gasoline under the influence of high temperature and pressure during
compression. Therefore, HCCI provides us with the best of both worlds.
Working of HCCI:
The working of HCCI is based on the four stroke Otto cycle. During the suction stroke,
gasoline and air mixture is introduced in the combustion chamber. It is
important to keep the air-fuel mixture much leaner in order to help in
effective combustion process to take place. A conventional S.I engine has a
stoichiometric air-fuel ratio of 14.7:1. But in a HCCI engine, the air-fuel
mixture ranges from 24:1 to 31:1 which makes it easier for the engine to self
ignite the charge.
Next follows the compression stroke. Unlike a conventional
S.I engine, HCCI engines usually have a higher compression ratio up to 16:1.
This helps in achieving the required temperature and pressure to auto ignite
gasoline.
Just like a conventional C.I engine, the combustion takes
place right at the end of compression stroke in a HCCI engine. The piston then
moves from T.D.C to B.D.C to complete power stroke. Exhaust stroke follows the
power stroke. The emissions are much lesser compared to both S.I and C.I
engines.
Advantages of HCCI:
·
Fuel consumption can be reduced by 15% compared
to S.I engine.
·
Emissions are lower due to lower peak
temperature.
·
It can run on gasoline, diesel and other
alternative fuels.
Disadvantages of
HCCI:
·
Higher cylinder pressure can damage the
cylinder.
·
Auto ignition can be difficult to control. It
can lead to excessive knocking.
·
Power generated is less compared to S.I engines.
·
Cold starting is difficult without a spark plug.
