Why Hybrid Cars Are Fuel-Efficient
Where does all that energy go to?
When fuel is burnt in a car's engine, most of the energy goes straight out of the exhaust pipe into the surrounding air and heats it up. A lot of it ends up heating up the engine itself which has then to be cooled by the radiator which again gives up its heat to the surrounding air. Only about 30 percent of the the energy is converted into propulsion power for the vehicle.
Losses at constant speed on level ground
When a car is travelling at constant speed on level ground, it requires power from the engine to keep it moving at that speed. This is to overcome the forces that are trying to slow the car down. The car experiences resistance from the moving parts of the engine, the transmission system and the wheels and also the rolling resistance of the tyres. Ever wondered why tyres get hot after the car has been moving for a while? Air resistance also tries to slow the car down. A measure of how much air resistance a car experiences is known as its drag coefficient. This air resistance increases as the square of the speed of the car. So a car travelling at 100 km/hr experiences four times the air resistance than if it was travelling at 50 km/hr.
When overcoming these resistances, engine, transmission, tyres and air resistance, heat is generated and dissipated into the surrounding air. This heat is obtained from the energy provided by the engine and ultimately from the fuel that is burnt. To minimize the energy lost this way, it is necessary to make sure the tyres are properly inflated and to avoid driving the car at excessively high speeds. It is estimated that at highway speeds, 60 percent of the energy generated by the engine is used up in overcoming air resistance.
Energy for acceleration
Let us look at what happens when a car travels from point A to point B. At point A, initially the car is at rest. It then accelerates and reaches a cruising speed of, say, 100 km/hr. That speed is maintained for a certain period of time until it reaches point B when the brakes are applied and it comes to rest again. When the car is moving, it has energy stored in it. This energy is referred to as kinetic energy which is provided by the fuel burnt in the engine. This energy is proportional to the weight of the car and to the square of the speed at which it is travelling. A car that weighs twice as much as another has twice the amount of kinetic energy at the same speed. A car that is travelling at 100 km/hr has four times the kinetic energy than if it is travelling at 50 km/hr, and hence uses up four times the amount of fuel during acceleration.
Energy lost in braking
What happens to all that kinetic energy if a car travelling at 100 km/hr is suddenly brought to rest for example if it hits a tree? All that energy is then used up to crumple the metal-work, and damage all the other mechanical parts of the car, not to mention injuries to the occupants. In the normal course of events, the brakes are applied when the car has reached its destination so that it can be brought gently to a dead stop. What happens to the kinetic energy in this instance? It all ends up in heat generated in the brakes.
Start-stop driving
Normal town driving uses up a lot of fuel. If we take, for example, a stretch of road that is about 5 km in length that has traffic lights spaced at intervals of half a kilometre, and if we assume that you have to stop at every one of those lights, then the car burns fuel to accelerate from rest to normal driving speed. The brakes are then applied to bring it to a stop at the next set of traffic lights before it has to accelerate again. This is repeated ten times and each time fuel is burnt to bring the car up to speed and when the brakes are applied, it all ends up in heating up the brakes which have to be cooled by the surrounding air. When you consider that a car travelling at 60 km/hr has sufficient kinetic energy to allow it to coast for 1 km on level ground before coming to a stop, you begin to realise just how much energy is wasted when the brakes are applied to bring that car to a stop at the traffic lights.
The Solution
What if most of that kinetic energy could be stored up when the car is brought to rest and then re-used to bring it up to speed again between traffic lights? Well, that is what a hybrid electric car does. It is called regenerative braking. The electric motor which drives the car now operates as an electric generator to slow the car down. It converts most of the kinetic energy of the car into electric energy which is then stored in the main battery. Instead of heating up the brakes, the kinetic energy can now be stored to be used later. This is the single biggest reason why hybrid electric cars are so fuel efficient. It also explains why, unlike conventional cars, hybrid cars return a higher mileage in town driving as compared to highway driving where a constant speed can be maintained for great distances and most of the fuel is used up to overcome the air resistance. Under highway driving conditions, hybrid cars have no advantage over conventional cars.
There are some other reasons for the better fuel performance of hybrid cars. The engine of a conventional car is almost always sized to meet the peak demand of the vehicle. In normal operation, the power of the engine is under-utilized. This leads to inefficient use of the engine and high fuel consumption. The engine of a hybrid electric vehicle, on the other hand is significantly smaller. During acceleration the additional power is provided by the battery through the electric motor. Besides, the engine does not need to be running all the time. When the battery is low the engine runs at optimum speed to charge it up. At this speed it will be running at maximum efficiency. There is also no need to idle the engine when the car is stationary, for example at traffic lights, and hence no fuel is burnt since the engine can be switched off.
To learn more about how hybrid-electric cars work the InsightCentral website contains an exellent write-up about it.
Some interesting facts about the Toyota Prius can be found here.