Diesel Engine Explained
#1
09-10-2007, 08:58 AM
Diesel Engine Explained
4 stroke Diesel Engine explained
written by Robert Bosch
The diesel engine has long been the workhorse of industry. Favored for their high torque output, durability, exceptional fuel economy, and ability to provide power under a wide range of conditions, diesels are dominant in such applications as trucking, construction, farming, and mining, and are used extensively for stationary power generation and marine propulsion.
Diesel engines are similar to gasoline engines in many ways. Both are internal combustion engines and most versions of both use a four-stroke cycle. But there are four fundamental differences:
The gasoline engine injects fuel to the air as it is drawn into a cylinder. The diesel engine draws air into a cylinder and injects fuel after the air has been compressed.
The gasoline engine ignites the fuel-air mixture with a spark. The diesel engine relies on high temperature alone for ignition. Since this high temperature is the result of compressing air above the piston as it travels upward, diesel engines are often referred to as compression-ignition engines.
The power output of a gasoline engine is controlled by a throttle, which varies the amount of fuel-air mixture drawn into a cylinder. A diesel engine does not throttle the intake air; it controls the power output by varying the amount of fuel injected into the air, thereby varying the fuel-air ratio. This is one of the primary reasons why diesel engines are more fuel efficient than spark ignition gasoline engines.
A gasoline engine runs stoichiometrically – the fuel-air ratio is fixed so that there is just enough air to burn all the fuel. A diesel engine runs lean – there is always more air than is needed to burn the fuel.
The main advantage of a diesel engine is its high thermal efficiency.1 Diesel engines can achieve thermal efficiencies in excess of 50%. The best gasoline engines are only about 30% to 33% efficient, and then only at wide throttle openings. As a result, diesel engines have better fuel economy than gasoline engines.
FOUR-STROKE CYCLE
By far the most common type of diesel engine used today has reciprocating pistons and uses a four-stroke operating cycle. In the first stroke (intake stroke), the intake valve opens while the piston moves down from its highest position in the cylinder (closest to the cylinder head) to its lowest position, drawing air into the cylinder in the process. In the second stroke (compression stroke), the intake valve closes and the piston moves back up the cylinder. This compresses the air and, consequently, heats it to a high temperature, typically in excess of 1000°F (540°C). Near the end of the compression stroke, fuel is injected into the cylinder. After a short delay, the fuel ignites spontaneously, a process called autoignition. The hot gases produced by the combustion of the fuel further increase the pressure in the cylinder, forcing the piston down (expansion stroke or power stroke). The exhaust valve opens when the piston is again near its lowest position, so that as the piston once more moves to its highest position (exhaust stroke), most of the burned gases are forced out of the cylinder.
written by Robert Bosch
The diesel engine has long been the workhorse of industry. Favored for their high torque output, durability, exceptional fuel economy, and ability to provide power under a wide range of conditions, diesels are dominant in such applications as trucking, construction, farming, and mining, and are used extensively for stationary power generation and marine propulsion.
Diesel engines are similar to gasoline engines in many ways. Both are internal combustion engines and most versions of both use a four-stroke cycle. But there are four fundamental differences:
The gasoline engine injects fuel to the air as it is drawn into a cylinder. The diesel engine draws air into a cylinder and injects fuel after the air has been compressed.
The gasoline engine ignites the fuel-air mixture with a spark. The diesel engine relies on high temperature alone for ignition. Since this high temperature is the result of compressing air above the piston as it travels upward, diesel engines are often referred to as compression-ignition engines.
The power output of a gasoline engine is controlled by a throttle, which varies the amount of fuel-air mixture drawn into a cylinder. A diesel engine does not throttle the intake air; it controls the power output by varying the amount of fuel injected into the air, thereby varying the fuel-air ratio. This is one of the primary reasons why diesel engines are more fuel efficient than spark ignition gasoline engines.
A gasoline engine runs stoichiometrically – the fuel-air ratio is fixed so that there is just enough air to burn all the fuel. A diesel engine runs lean – there is always more air than is needed to burn the fuel.
The main advantage of a diesel engine is its high thermal efficiency.1 Diesel engines can achieve thermal efficiencies in excess of 50%. The best gasoline engines are only about 30% to 33% efficient, and then only at wide throttle openings. As a result, diesel engines have better fuel economy than gasoline engines.
FOUR-STROKE CYCLE
By far the most common type of diesel engine used today has reciprocating pistons and uses a four-stroke operating cycle. In the first stroke (intake stroke), the intake valve opens while the piston moves down from its highest position in the cylinder (closest to the cylinder head) to its lowest position, drawing air into the cylinder in the process. In the second stroke (compression stroke), the intake valve closes and the piston moves back up the cylinder. This compresses the air and, consequently, heats it to a high temperature, typically in excess of 1000°F (540°C). Near the end of the compression stroke, fuel is injected into the cylinder. After a short delay, the fuel ignites spontaneously, a process called autoignition. The hot gases produced by the combustion of the fuel further increase the pressure in the cylinder, forcing the piston down (expansion stroke or power stroke). The exhaust valve opens when the piston is again near its lowest position, so that as the piston once more moves to its highest position (exhaust stroke), most of the burned gases are forced out of the cylinder.
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