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?