In a petrol engine, when the piston reaches the top compressing the air-fuel mixture, the spark plug, which protrudes into the combustion chamber, ignites the fuel. Hence petrol engines are also called spark ignited (SI) engines.
In a diesel engine there is no spark plug; instead, combustion is caused by compressing the air inside the cylinders. Air heats up when it is compressed and if the compression is very high, the air temperature in the cylinder reaches a point that when fuel is squirted into it, it ignites at once, without any need of a spark plug. Instead of a spark plug, a diesel combustion chamber has fuel injected into it precisely measured for combustion. Hence a diesel engine is known as a compression ignition (CI) engine.
A car is called a hybrid when power is generated from two or more sources. It could be a combination of an IC engine and an electric motor and/or flywheels (KERS system). An electric hybrid can further be classified into three sub-categories - micro hybrids, mild hybrids and full hybrids
Micro hybrids are cars running on IC engines with a start-stop mechanism. The engine shuts off when the car is idling and starts once the clutch is engaged.
Hybrids with small electric motors that provide start-stop features and supply extra power from the electric motor during acceleration are called mild hybrids. The battery gets replenished during deceleration/braking (regenerative braking). The electric engine cannot be used to drive the car by itself. The Honda Civic hybrid launched in 2008 is a mild hybrid.
A car that can be propelled using the electric motor by itself and can switch to the ICE when more power is required is called a full hybrid. The Toyota Prius is a full hybrid; the electric motor is used at low speeds in start-stop traffic situations in the city and switches to the gasoline motor at higher speeds. It has a clever power split device that uses the gasoline engine, electric motor and generator together. Power can be drawn by either of the two motors or both at the same time. The batteries do not have to be recharged since the generator maintains a constant power level in them.
Direct Injection Engines
Conventional engines mix air and fuel in the intake port and force the mixture into the cylinders for combustion. In direct injection engines, fuel is sprayed into the cylinders via injectors and air enters from the intake port. Like diesel engines, a common rail fuel line supplies petrol to the injectors which is then forced into the combustion chamber under high pressure. The advantage of direct injectors is that the amount of fuel can be precisely controlled by the ECU in accordance with the power demanded. In short – better fuel efficiency and higher power output.
Fuel is injected into the cylinders at as many as 15 times per second. With the help of the ECU, the amount of fuel injected is controlled such that the air-fuel mixture at the top of the piston is at a 14:1 ratio while the layers below it are much leaner, and could go as high as 40:1 or more. This is called stratified fuel injection seen in the Mercedes C200 CGI. In India though, the C-class uses only direct injection since our fuel quality does not allow a stratified charge to run.
A balance between power and efficiency is thus obtained by controlling the number of injections and the amount of injection during every induction stroke.