Engine load and Speed

For developing higher power more fuel is injected in the diesel engine thereby reducing excess air, which results in higher combustion and exhaust temperatures. A typical dependence of NO on air-fuel ratio i.e., engine load has been shown in Fig. 3.4. Dependence of smoke on overall fuel-air ratio for a direct injection diesel engine is shown on Fig. 3.8.

With increase in engine load (increase in fuel-air ratio);

• NO_X and soot emissions increase.

•   With increase in engine load as the combustion temperatures increase and oxidation environment for CO is more favourable, CO emissions decrease until excess air reduces to about 30 percent . With further increase in load and fuel-air ratio, CO emissions start increasing again and rise sharply as still more fuel is injected to increase engine power output.

•   At maximum load, NO_x , CO and soot are also at their maximum level. HC however, reduce with increase in engine load as higher gas temperature lead to an increase in the oxidation rates.
•   Engine brake thermal efficiency increases with engine load because the ratio of friction to brake power goes down. Interaction among these factors results in lowest value of brake specific fuel consumption (BSFC), and optimum value of brake specific nitrogen oxides (BSNOx ) and particulate emissions at an intermediate load.

The variable speed engines are designed to give lowest fuel consumption at about 2/3rd of maximum speed at which heavy duty engines are normally operated. In turbocharged engines, the boost pressure is reduced at low engine speeds resulting in higher fuel-air ratio. At high speeds pumping losses increase and cooling decreases. The coolant and residual gases are hotter as the speed increases. Both these factors increase NO _x at high engine speeds. The HC and PM have an optimum at an intermediate speed because time available for oxidation decreases at the higher engine speeds.