Types of Diesel Engines and their Selection for Diesel Power Plants
Diesel engines are generally classified as :
- Four stroke diesel engine in which one power stroke is developed every two revolutions of the crankshaft.
- Two stroke diesel engine in which we get one power stroke in every revolution of the crankshaft.
- Two stroke engines are more compact and easy to start. These have low weight to power ratio with higher mechanical efficiency and low capital cost compared to four stroke diesel engine.
- However, the fuel and thermodynamic efficiency of two stroke engines is less compared to four stroke engine. Apart from this, the two stroke engines require comparatively more cooling water, consumes more lubricants, has more exhaust emissions with poor scavenging efficiency.
- The selection of engine size depends on the application and its capacity of the power plant. Their operation at part loads upto 80% are uneconomical.
- Small sized plants are preferred operating on four stroke cycle for application of automobiles and DG sets. However, for large power plants the two stroke diesel engines are preferred.
- Their plant capacity is estimated by having the survey of essential of connected loads needed to be supplied under emergency, the demand factor and the diversity factor.
- Then the required capacity of the plant is given by the equation :
Capacity of plant = Connected load x Demand factor Diversity factor
- Further the size and number of units of the diesel engines are worked out based on the generation cost and the overall plant economics as compared to the other type of plants. The plants below 3 MW are used as standby plants and between 3-25 MW as base load plants.
- These plants generally use four stroke diesel engine. The plants in capacities above 10 MW use two stroke diesel engines as base load plants.
- These engines apart from diesel oil can also run on other fuels also. Now a days dual fuel engines are also used working on gas as the main fuel and oil for start of combustion.
- These use leaner mixtures of air and fuel. These engines are economical and provide much less emissions.