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66 An Introduction t o Enqine Testinq and Development
oxygen. This weight of air is called the stoichiometric airlfuel ratio. The gasoline engine operates with an airlfuel mixture that is very near the stoichiometric aidfuel ratio. It is necessary to have this ratio because an airlfuel ratio much greater than stoichiometric is difficult to ignite with a spark plug, and one much smaller than stoichiometric (fuel rich) is inefficient. In the diesel engine, the fuel is injected into the combustion chamber at a point near the end of the compression stroke, and the fuel ignites spontaneously. As mixing occurs between the fuel and air, burning continues. This process is extremely heterogeneous. Soot (black smoke) is formed during combustion because some of this fuel has insufficient oxygen for complete combustion. As more fuel is injected, more and more soot is produced. Hence, the airlfuel ratio of the diesel engine must always be higher than stoichiometric to prevent excessive amounts of soot. To reduce the amount of soot means that less fuel is present in the diesel engine cylinder than in a cylinder of an equivalent gasoline engine, and the diesel engine power therefore is reduced. For the same swept volume, current diesel engines can use only 70 to 80% of the fuel used by a gasoline engine. (This is not the case in turbocharged applications.) In diesel engines, the utilization factor varies with the combustion systerdfuel delivery system. For example, a diesel engine with a pre-combustion chamber system (indirect injection [IDI]) has a utilization value of approximately 80%, whereas a normally aspirated direct injection (DI) system, which is used mainly for large trucks, has values of approximately 50 to 70%. Direct injection systems still are not as well developed as ID1 pre-combustion systems, but developments are moving forward at a rapid pace.
Another reason for the output of a diesel engine being less than that of a gasoline equivalent swept volume engine is that gasoline engines can operate at high speeds because the combustion rate increases with the engine speed. On the other hand, die- sel has a reduced combustion efficiency at high speeds because of the longer ignition delay, slow mixing of the mixture, and longer injection duration (in terms of crank angle). Note that this is not as relevant with electronic diesel injectors; those based on the piezo crystal expansion with current have infinitely fast response. The international governmental smoke limits are difficult to meet at high speeds. The diesel engine has a very high compression ratio, and the energy required to rotate the crankshaft (i.e., fric- tion horsepower) is larger than that of the gasoline engine. The friction loss is large in proportion to the engine rotational speed. Thus, when the speed is increased to boost the output, the friction losses increase and thereby negate the increase in output. The maximum speed that a current Formula 1 engine can reach is 17,000 revlmin; a diesel can reach only approximately 5000 revlmin.
Advantages of Diesel Engines
We have reviewed the many disadvantages of diesel engines, so one might wonder why the diesel engine is used? The answer is that a diesel engine has very good fuel consumption; indeed, it is so good that it offsets the many disadvantages.
When comparing fuel consumption per horsepower per hour at the maximum output between the best diesel and the best gasoline engines of similar swept volumes and valve timing configurations, it will be found that the diesel engine uses only approximately 70% of the fuel that the gasoline engine uses. In part-load condition-the condition in which on-road application is normal automobile driving, idle to half throttle-the fuel consumption of a diesel engine can be as little as 60% of that of the gasoline engine.
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Why Does the Diesel Engine Have Such Good Fuel Consumption ?
The good fuel consumption of the diesel engine is due to the high compression ratio required for self-ignition and the reduced pumping losses because the diesel has no induction air throttle plate. The higher the compression ratio, the better the thermal efficiency. The gasoline engine cannot accommodate high compression ratios because of the onset of the knocking phenomenon.
The theoretical thermal efficiency is given by
where E = ratio of compression and expansion (compression ratio) Q = ideal gas
k = specific heat at a constant pressure/specific heat at a constant volume That is, the theoretical thermal efficiency is related only to the compression ratio and K, and it corresponds to the efficiency of the Carnot cycle active between temperatures T I and T2 or T3 and T4.
As long as fuel economy is measured using fuel volume, diesel fuel has another advan- tage. Diesel fuel has a specific gravity that is approximately 10% higher than that of gasoline, that is, one liter of diesel fuel is 10% heavier than one liter of gasoline. The amount of energy in a specific weight of diesel fuel thus is 10% greater than that of gasoline. When fuel economy is measured by volume (i.e., kilometers per liter or miles per gallon), diesel fuel would produce 10% greater h e 1 consumption than gasoline, even if the engines otherwise were identical.
Diesel uses an airifuel mixture with approximately 40% excess air; the gasoline engine runs with a stoichiometric mixture. The leaner mixture results in higher efficiency because with a lean mixture, the combustion temperatures are lower with sufficient oxygen. Lower combustion temperatures reduce heat loss.
A better fuel consumption signifies that the energy in the fuel is used efficiently and at a lower combustion temperature due to a higher airlfuel ratio, which reduces the heat quantity released to the coolant. This reduced heat means that the radiator can be downsized, and the cooling fan can be reduced in size. In some gasoline-to-diesel conversions, the radiator can be reduced by more than 35%.
A key advantage of the diesel engine is its durability, resulting in a longer service life as a direct result of the robust and heavy structure required to sustain the high combus- tion pressures.
As we have discussed, the peak cylinder pressure of the diesel engine is more than 1.5 times that of the gasoline equivalent. If the same stress is put on a structure but at a higher pressure, then that structure must be thicker. For example, consider a round bar with a diameter D loaded as shown in Figure 4.8.
The stress is proportional to 1/D 3 , while the deflection is proportional to 1 1 ~ ~ . Namely, to keep the stress the same, the increase in the diameter must have a greater rigidity that is proportional to the increase of the diameter. If a diesel engine is designed for
68 An Introduction to Enqine Testinq and Development
Figure 4.8 Bending moment.
a higher cylinder pressure, its deflection would be similar to that of a gasoline engine designed for a lower cylinder pressure. However, note that higher crankcase rigidity always gives better durability.