TECH ED’S CHOICE
If we consider different powertrains driving a similar vehicle at the same speed, should the fuel consumption not be equal, as the energy needed to propel the vehicle stays constant? This would include petrol, diesel, naturally aspirated and turbocharged units of varying capacities. In my opinion, a smaller-capacity engine is not always more fuel efficient in day-today driving. Do you support my views? JEREMIAH MNISI Hazyview
Great question, Jeremiah. When you consider the movement of a vehicle from an energy point of view, you are correct. It takes a set amount of energy to propel a vehicle at a set speed. If the vehicle specification and mass is fixed but the powertrain is varied, as you described, the energy requirement does not change.
What’s left out of the equation is the efficiency of the powertrain in converting the energy from petrol, diesel or battery to the useful motive energy needed (which is the same amount, as explained).
An efficiency of 30% means that only 30% of the available energy in the fuel is converted to motive force. Depending on the technology used for each type of powertrain, the efficiencies can vary dramatically within each type but, for the purpose of this discussion, we’ll generalise.
PETROL ENGINES: 30% EFFICIENT
Petrol engines run at the ideal stoichiometric ratio of 14,7:1 air-to-fuel ratio. This means that, during part-load conditions, the incoming air needs to be throttled by a throttle valve resulting in pumping losses. By reducing the engine capacity, the engine needs to work harder to produce the required power (energy) and it means the throttle plate is open wider more often, resulting in considerably less pumping losses. Smaller-capacity engines also have less engine friction. Adding a turbo in downsizing applications increases the efficiency further by harnessing the normally wasted exhaust energy to compress the intake air. Obviously, this also results in a performance benefit to offset the capacity downsizing.
DIESEL ENGINES: 40% EFFICIENT
Diesel engines get a head start when it comes to fuel consumption because diesel fuel has slightly more energy per litre than petrol. Furthermore, a diesel engine can run with excess air as a fixed air-fuel ratio is not needed and therefore little pumping losses are present. The compression ratio is linked to efficiency and diesel engines run at higher ratios to achieve the temperatures and pressures needed in the combustion chamber for compression ignition (in contrast to a petrol engine, which relies on spark ignition via the spark plugs). Efficient turbocharging and downsizing is possible with diesel engines, too.
ELECTRIC POWERTRAINS: 90% EFFICIENT
Electric motors are by far the efficiency winners when it comes to converting chemical potential energy in the battery (electricity) to motive force. Figures higher than 90% are common. The drawback is the energy density of a battery pack does not compare to the energy density of fossil fuels, meaning driving range will always be an issue because batteries with enough energy to compete with fossil-fuel vehicles are heavy and expensive.
The above is a good indication of the direct running costs of a vehicle with different powertrains. In general, electric vehicles have the lowest energy cost followed by diesel vehicles and then petrols.