Environmental Engineering Reference
max p bmep
Figure 8.5 A diagram of contours of engine efficiency η e as a function of engine speed N and brake mean
effective pressure for an SI engine. Contour values are expressed as a percent of the maximum value.
For gasoline of SG
the LHV is 41.18 MJ/kg or 35 MJ/L. 10 Converting to thermal efficiency by equation (8.8), the best
SI engine efficiency is about 31% while that of a CI engine is 44%.
The engine efficiency
72, the LHV is 43.55 MJ/kg or 31.6 MJ/L, while for diesel fuel of SG
η e is a function of the engine speed N and brake mean effective pressure,
as demonstrated in Figure 8.5. Its peak value occurs at about 35% of the maximum engine speed
N m and 80% of the max p bmep , where the engine power is about 35% of the maximum power. The
efficiency declines rapidly with decreasing p bmep but less so with increasing speed. This decrease
of efficiency is a consequence of the relative increase of engine friction and flow losses, compared
to engine output, as p bmep and engine power are reduced below the optimum value. Because a
road vehicle engine must provide the full range of its power over the speed range of the vehicle, it
cannot operate at maximum efficiency all the time. Nevertheless, by proper matching of the engine
to the vehicle, it is possible to minimize the fuel consumption needed to meet a particular vehicle
VEHICLE POWER AND PERFORMANCE
Moving a vehicle along a highway requires the expenditure of mechanical power to turn the wheels.
Part of this power is needed to overcome the drag force exerted by the air on the moving vehicle.
Another part is needed to counter the resistance of the tires moving over the ground, called the
rolling resistance. If the vehicle is climbing a hill, additional power is needed to lift it vertically
10 Diesel fuel has more heating value per unit volume than gasoline, by about 11 %. Where vehicle fuel
efficiency is measured in terms of miles per gallon or kilometers per liter, a diesel-powered vehicle has an
inherent 11 % advantage, other things being equal.