The Diesel Engine

The Combustion Cycle The four-stroke combustion cycle of the diesel

engine is composed of the intake stroke, compression stroke, power stroke, and the exhaust stroke.

The diesel engine operates through the motion of compression pistons in inside the cylinders of the engine.

Pistons inside the cylinders are connected by rods to a crankshaft.  As the pistons move up and down in their cylinders, they cause the crankshaft to rotate. 

The crankshaft’s rotational force is carried by a transmission to a drive shaft, which turns axles, causing the wheels to rotate.

The Intake Stroke During the intake stroke, the

intake valve opens as the piston moves down to allow air into the cylinder.

The air is pushed by atmospheric pressure into the cylinder through the intake valve port.

The Compression Stroke After the piston reaches its lower

limit, it begins to move upward and as the piston moves up, the intake valve closes.

The exhaust valve is also closed, so that at this point in the cycle, the cylinder is sealed.

As the piston moves upward, the air is compressed and the compression of the air causes the pressure and the temperature of the cylinder to increase.

The Power Stroke As the piston reaches maximum

compression of the air, diesel fuel is injected to the combustion chamber filled with compressed air.

The heat of the compressed air ignites the fuel spontaneously at the self-ignition temperature of the fuel.

As the cylinder pressure increases, the piston is forced down into the cylinder.

The power impulse is transmitted down through the piston, through the connecting rod, and to the crankshaft, which is rotated due to the force.   

The Exhaust Stroke As the piston reaches the bottom

of its stroke, the exhaust valve opens.

As the piston moves up on the exhaust stroke, it forces the burned gases out of the cylinder through the exhaust port.

Then as the piston reaches the top of its stroke, the exhaust valve closes, and the intake valve opens.

The cycle repeats again with the intake stroke.

Efficiency One positive aspect of the diesel engine is

that they are thermally efficient. The improved efficiency is caused by the

relatively high compression ratios The diesel engine is 54% thermally

efficient, while gasoline engines are only 34%.  

As a result of diesel engines thermally efficiency, they are able to achieve better gas mileage because they produce greater horsepower output for fuel intake.

Fuel Economy One main reason for the diesel engine’s

excellent fuel economy is that it burns far more air than fuel.

In a gasoline engine, it’s air intake is carefully restricted and controlled by the carburetor for a 15:1 air to fuel ratio.

However, in the diesel engine, the air intake is unrestricted. 

Durability Diesel engines are also capable of running high miles

without major engine work. Truck diesel engines can run 250,000 or even 500,000

miles without major engine work. In 1978, Mercedes-Benz launched a “Great Diesel

Search” to find the highest mileage diesel in the United States. Robert O’Reilly drove his 1957 Mercedes-Benz 180D

1,184,000 miles, 750,000 on its original engine. Another owner drove his 1968 Mercedes-Benz 220

D 912,000 miles, 902,000 miles on its original engine.

Compression Ratio The diesel engine compresses at a very high ration of

14:1 to 25:1. The higher the compression ratio, the more power is

generated. The main limiting factor to compression ratio is based

on the knock limits of the fuel. Knock is the term used to describe the auto ignition

that occurs when a fuel ignites because the pressure in the cylinder is such that combustion occurs.

The knock limit of the fuel is determined by the point at which the engine begins to shake.

The higher the knock limit, the higher the compression ratio, the more power is generated.

Advantages of a Diesel Engine Low maintenance, greater efficiency, high power

output, and better fuel economy under all types of loads.

Does not require a spark plug to ignite fuel, it relies on the spontaneous combustion through the heat of compression to ignite the diesel fuel.

Because of this type of combustion, a diesel engine are built more ruggedly and heavily built than the gas engine.

The ruggedness of the diesel engine gives a two to three times longer life than the gas engine, which has a life of around 100,000 miles.

Disadvantages Components of diesel engines are usually heavier

that those of gasoline engines because of the additional structural strength needed to obtain the higher compression ratio and power output.

They can emit large amounts of ozone-forming constituents and particulates.

Because of diesel engines’ weight and compression ratio, they tend to have lower maximum RPMs than gasoline engines. This makes diesel engines has high torque rather

than high horsepower, and this tends to make cars with diesel engines slow in terms of acceleration.

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