steve@ncsc.navy.mil (Mahan) (11/09/89)
From: steve@ncsc.navy.mil (Mahan)
Subj: Fuel injection systems
In issue 28 in the discussion of fuel injection it was stated that the fuel
is injected directly into the engine cylinder. In general this is true only
for Diesel engines.
Most gasoline powered engines that use fuel injection place the injectors
somewhere outside the cylinders in the intake air stream, either in the place
of a conventional carburetor (throttle body injection) with the injectors
feeding the cylinders through a common manifold or in the intake ports of each
cylinder. Injectors in the intake ports may be set up to spray fuel constantly
or to provide pulses of fuel synchronized with the operation of the intake
valve. The only gasoline engine that I am aware of that used direct cylinder
injection (other than racing engines) was the Mercedes-Benz 300 SL. This
passenger car engine had a 3 liter displacement and produced 154 HP from 6
cylinders in 1954.
The reasons for using direct cylinder injection in a gasoline engine
involve better cylinder filling due to the cooling effect of the injected fuel
charge on the air in the cylinder. This tends to increase the cylinder vacuum
and further accelerate the air in the intake port and runner, providing better
cylinder filling and consequent increased power. The main disadvantage of this
system is the need to provide a high pressure (3000 psi) Diesel type injection
system capable of pumping gasoline (2-3,000 1989 dollars). Most Diesel
injection pumps rely on the fuel for lubrication and the modifications to
deal with gasoline and its poor lubricating properties are expensive.
Stephen Mahan
Naval Coastal Systems Center
Panama City, FL 32408-5000
ARPA: steve@ncsc.navy.mil
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