Diesel Engines 101
Every discussion must start somewhere. This one will start by
assuming the reader understands basic gasoline engines. If you
are not familiar with gasoline engines, refer to an online source, such
as: http://en.wikipedia.org/wiki/Four-stroke_engine.
This discussion will be limited to the type of Diesel engines used in
Kioti Tractors. These are four-stroke Diesels with a Bosch-style
mechancal in-line injection pump and mechanical injectors.
There are many similarities between gasoline and Diesel engines.
Both have pistons that connect to crankshafts with connecting
rods. Both have intake and exhaust valves
controlled by a cam shaft. From here on I'll focus on the
differences.
Technically, a throttle is the restriction that limits the amount of air going into the engine. Diesels don't have a throttle.
This means that a great deal of air gets sucked into the engine and
there is very little manifold vacuum. In fact, the manifold
pressure in turbo Diesel engines is positive (above atmospheric).
This also means that Diesel engines run very lean at all but the
highest power settings. This "excess air" during combustion
allows more complete combustion producing very low levels of carbon
monoxide (compared to gas engines which operate close to stochiometric
fuel-air ratios).
Diesels use a much higher compression ratio.
Instead of 8:1 to 11:1, which is typical of gasoline engines; Diesel
compression ratios are typically greater than 20:1. This
means several things. First, all parts of the engine need to be
stronger to endure the higher forces. Second, the temperature and
pressure of the compressed air charge is so high that the fuel will
burn spontaneously without a spark. This is the reason that
spark-ignited gasoline engines cannot use higher compression
ratios. Instead of ingesting an air-fuel mixture through the
intake valves, Diesel engines spray atomized fuel directly into
the combustion chamber just as each piston approches the top of the
compression stroke. Finally, higher compression ratios produce
higher efficiency (in addition to the higher energy content of Diesel
fuel compared to gasoline).
Diesel injection systems have a hard job. The injection pump
(IP) must deliver the same precise amount of fuel to each cylinder at
the same point in the compression stroke, otherwise the engine will run
rough. Futhermore the IP must
deliver this precise amount of fuel under several thounds PSI
pressure. Diesels use
very little fuel at idle so the IP must be capable of delivering tiny
amounts of fuel as well as large amounts at high power settings.
Unlike conventional gasoline engines where the gas pedal directly
controls the throttle plate, Diesel engines use a centrifugal governor
suct that "throttle" position causes the IP to deliver enough fuel to
maintain a certain RPM. If the engine load increases or
decreases) the IP changes the amount of fuel supplied to maintain the
commanded RPM.
Mechanical Diesel injection systems use mechanical injectors
which seal tight until the fuel pressure (from the IP) reaches a preset
"pop" pressure (thousands of PSI) which forces the injector open to
deliver a finely atomized fuel spray into the combustion chamber.
This fuel mist ignites and burns spontaneously in the hot, high-pressure air.
Kioti Diesels are the indirect injection type with prechambers and glow plugs.
A prechamber sits in the engine head on top of each cylinder.
Prechamber designs tend to be quieter and smoother than "direct
injection" designs (i.e., no prechamber or glow plug) but are somewhat less
efficient. When an engine is very cold, the prechamber mass will
cool the compressed air below the point where fuel will ignite.
So a glow plug provides additional heat in the prechamber before and
for a short time after starting.
So what does this mean to owners/operators?
- Good air filtration is critical. Huge
amounts of air move through Diesel engines and so will entrained dust,
dirt, and sand unless they are filtered out. Also, a clogged air
filter will limit the amount of air going into the engine which will
reduce available power and may cause hard starting.
- Good fuel filtration is critical. There are
small passages and close tolerences in the IP and injectors that will
wear or malfunction unless all solids are filtered out of the fuel.
- The starter (and battery) must be capable of
cranking the engine at a fast rate to achieve temperatures required to
ignite the fuel.