Advantages Of Diesel Engines
When comparing fuel consumption per horsepower/hour at the maximum output between the best diesel and the best gasoline engine, the oil burner uses only about 70 percent of the fuel the gasoline engine does. Partial load conditions are what most engines-either gas or diesel-operate under, and during this scenario, a diesel will burn only 60 percent of the fuel that a gasoline engine would.
The reason that a diesel engine has such good fuel economy lies in the high compression ratio required for auto-ignition. The higher the compression ratio, the better the thermal efficiency. A gasoline engine can't utilize a diesel-like compression ratio because of the fuel's inability to resist auto-ignition, or what is commonly known as detonation or ping.
The Ford Power Stroke was the first light-duty diesel that incorporated electronic controls over the fuel delivery. The same basic injector is used in the International DT-530 engine. Many enthusiasts install DT-530 injectors in modified Power Strokes because they have the ability to pass more fuel.
As long as fuel economy is measured using fuel volume, diesel fuel has another advantage. Diesel fuel has a specific gravity about 10 percent heavier than gasoline. In other words, one gallon of diesel fuel is 10 percent heavier than a gallon of gasoline. The amount of energy in a specified weight of diesel fuel is almost the same as that of gasoline, so the amount of energy in a gallon of diesel fuel is 10 percent greater than gasoline.
Another important reason for higher efficiency in diesel engines is that, while diesel uses an air/fuel mixture with around 40 percent excess air, the gasoline engine runs with a stoichiometric mixture (less air for the fuel consumed). The leaner diesel mixture results in higher efficiency because the combustion temperatures are lower with less fuel when sufficient oxygen is supplied. Lower combustion temperature reduces heat losses in the engine, and more of the energy from the fuel is used to expand against the piston. This reduced heat means the radiator can be downsized (in relation to a gas engine producing the same amount of power) and the cooling fan can be made smaller. In many direct-injection diesel applications, the radiator size can be reduced by more than 35 percent.
Shortcomings of Diesel Engines
When compared to a gasoline engine of the same displacement, a diesel's biggest disadvantage is weight. Usually, diesel engines have a weight per horsepower of one and a half to three times more than gasoline engines. The reason for the increased weight is that a diesel engine relies on a high compression ratio.
On the Power Stroke, the injector resides in the cylinder head between the two valves.
In a diesel, the air is heated by the squeezing action of the piston and the fuel is ignited by being sprayed into the heated air. As a result, its combustion pressure is much higher than a gasoline engine; a robust structure is required to withstand the high pressure. The combustion pressure of a naturally aspirated (no turbocharger) diesel engine is about one and a half times higher than that of a naturally aspirated gasoline engine.
Another reason the output of a diesel engine per cubic inch is less than that of the gasoline engine is because a spark-ignition engine can operate at a higher rate of speed. This is due to the combustion burn rate of gasoline being faster. Diesel fuel has a reduced combustion efficiency at high speeds because of the longer ignition delay, longer injection duration (in crankshaft angle degrees), and from the slow mixing rate. In diesel engines, the government-mandated smoke limits are very hard to meet at high speeds since the engine doesn't have enough cylinder event time to burn all of the fuel. Since the diesel engine has a high compression ratio, the energy required to revolve the engine itself, or what is known as friction loss, is greater than that of the gasoline engine. Therefore, when speed is increased to boost horsepower output, the friction loss raises enough to offset the output component of the engine. For a production-style gasoline race engine, a practical maximum speed is around 10,000 rpm. A similar diesel engine would only be able to achieve approximately 5,000 rpm.
The Four-Strokes of Diesel
All modern light- to medium-duty diesel engines sold for use in a motor vehicle in the U.S. are of the Otto cycle (four-stroke) design. In the four-stroke diesel cycle, the following sequence of events is continuously repeated when the engine is running:
1. The Intake Stroke:
During this time, the piston is moving downward and only air is drawn into the cylinder. This occurs on a normally aspirated engine as a result of the difference between atmospheric pressure and the depression created by the piston and the piston ring seal. On forced-induction applications, the induction process is aided by the external air movement caused by the turbocharger or supercharger.
2. The Compression Stroke:
The piston is moving upward in the bore and compresses the air into the very small volume of the combustion chamber and raises its temperature high enough to ensure auto-ignition of the fuel charge. This demands a compression ratio higher than that employed in a gasoline engine. The air is heated by the friction of the molecules being packed into a very small area.
3. The Power Stroke:
The proper name for this event is "expansion" because of the flame expanding against the piston. Created by the diesel fuel charge injected into the combustion chamber and being mixed with the very hot air, the ignited gases expand and are responsible for the transfer of chemical energy to mechanical energy.
4. The Exhaust Stroke:
During this step, the byproducts of combustion are purged from the cylinder and combustion chamber. First, by the pressure differential between the bore and the exhaust port of the head (this is identified as blow down), and then by the movement of the piston upward. The inert gases exit the cylinder bore through the exhaust port in the cylinder head.