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FOUR-STROKE ENGINE OVERVIEW
(SPARKPLUG .VS. GLOW PLUG)

Are glow plugs in a diesel engine for the same purpose as the sparkplugs in a gasoline engine?

The short answer is no.  In a gasoline engine (Otto-cycle) a sparkplug causes ignitioni.  In a diesel engine (diesel-cycle) adiabatic heating of air causes ignitionii (compression-ignition).  The reason I give the short answer is that I have observed persons read similar articles to this one and come away with an affirmative answer.

GASOLINE ENGINE (sparkplug)

In a four-stroke gasoline engine the fuel and air is mixed somewhere before the intake valves (see DGIiii).  This mixture is pulled into the engine cylinder when the intake valves open and the piston starts down away from the cylinder head for the intake-stroke.  At the bottom of the intake-stroke, the intake valves close and the piston travels back towards the head, compressing the air fuel mixture (compression-stroke).  At an appropriate point near the top of the piston travel (Top Dead Center – TDC of the compression-stroke), a high voltage pulse is applied to the center electrode of the sparkplug causing a spark to jump the gap to the outer electrode.  This spark ignitesiv the air fuel mixture, releasing the chemical energy of the fuel as heat and a large volume of gas, driving the piston back away from the cylinder head for the power-stroke.  At the end of the power-stroke the exhaust valves open and the piston travels back towards the head expelling the combustion gases for the exhaust-stroke.  At the end of the exhaust-stroke, the exhaust valves close and the process starts all over.  In a gasoline engine, the sparkplug ignites the air fuel mixture at the beginning of every power stroke (similarly in a two-stroke gasoline engine).

DIESEL ENGINE (glow plug)

In a four-stroke diesel engine, the air enters the piston through the intake valves devoid of fuel.  As the piston moves away from the cylinder head, the air is drawn into the piston for the intake-stroke.  At the bottom of the intake-stroke, the intake valves close and the piston travels back towards the head, compressing the air (compression-stroke).  During the compression-stroke the air is heated to a temperature higher than the autoignition temperature (624 F) of the diesel fuel by means of adiabatic compression heating.  At an appropriate point near the top of the piston travel (TDC of the compression-stroke), a high-pressure mist of diesel fuel is sprayed into the cylinder (or a pre-chamber).  The hot air causes the diesel fuel mist to spontaneously combustv.  Combustion releases the chemical energy of the fuel as heat and a large volume of gas, driving the piston back away from the cylinder head for the power-stroke.  At the end of the power-stroke the exhaust valves open and the piston travels back towards the head expelling the combustion gases for the exhaust-stroke.  At the end of the exhaust-stroke, the exhaust valves close and the process starts all over.  The hot air provided by the compression-stroke ignites the fuel mist at the beginning of each power-stroke (similarly for a two-stroke diesel engine).

When the engine is cold, the heat loss to the block is so high that it is not possible to heat the air to the autoignition temperature during the compression-stroke.  To get around this, a cartridge heater called a glow plug is placed in the cylinder.  Temperature sensors detect when the engine is too cold to create enough heat by adiabatic compression heating to exceed the autoignition temperature and the glow plug is activated for start up.  Glow plugs are only used in a cold engine condition at start up.


 

i

Notice this is combustion, i.e. burning, not explosion.

 

ii

ibid

 

iii

DGI – Direct Gasoline Injection, is beyond the scope of this article.  In a direct gasoline injection engine, the high-pressure fuel is added (sprayed) into the cylinder at an appropriate point in the compression stroke before the sparkplug is fired.  DGI has performance advantages.  On the downside, NOx emissions can be higher without mitigation efforts, and the fuel system is more complex and more expensive.  Here are several articles that delve into DGI:
http://cars.about.com/od/thingsyouneedtoknow/a/directinjection.htm
http://en.wikipedia.org/wiki/Gasoline_direct_injection

 

iv

ibid i

 

v

ibid i


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