Do hybrid engines improve fuel efficiency?

By Chad Lückhoff

The very point of a hybrid engine is to be more fuel efficient, but the fact remains that not all hybrid systems are created equally. Some will favour electric use while other will favour the petrol engine – this is mostly decided by what type of car it is and its intended use. There's the last batch of super / hyper-cars that are technically hybrids, but offer less in the way of efficiency and rather use the electric motor assistance to boost their power output and speed.

To understand this, one needs to know a little about hybrid power. The name ‘hybrid’ suggests that there is an amalgamation, two entities joined to create one, carrying characteristic of both insertions. Hybrid cars come in a variety of different forms and setups, but the premise remains largely the same: both electric and fossil fuel propulsion. The aim here is to fill in the gaps where a fossil fuel (petrol or diesel) engine is its most inefficient.

The internal combustion motor by its very nature, is inefficient and wasteful. More that 70% of the energy created in the combustion cycle is wasted in heat. That means that at any given time, an Otto Cycle (4-stroke internal combustion) engine is no more than 30% effective. That's a lot of wasted energy! Over the years, many have tried to harness this wasted energy to make the common engine more efficient, most with little success. Turbocharging manages to make an engine more efficient by using wasted gasses to power a turbine that feeds additional air to the engine. In theory, the turbocharged engine uses less fuel than a non-turbo / naturally aspirated engine.

The Otto Cycle engine has the inherent flaw of also only being efficient at one particular rev range with anything above or below that becoming more wasteful as it strays from the sweet spot. This has to do with air/fuel mixture, timing and the camshafts. However, because we need to vary our speed, the engine speed needs to change and we thus also need gears, to keep the engine as close to this optimal RPM, for as long as possible.

To fill in the gaps where the Otto Cycle engine performs poorly, engineers have tried to apply electrical power to share some of the propulsion duties. In many non-switchable hybrid cars, the electric motor will help with the pull away, coasting and in between gear changes – areas where the internal combustion engine can be best assisted. Full bore acceleration is better with a petrol engine as this strain placed on an electric motor would drain the batteries rather quickly.

In many consumer hybrids, the Toyota Prius for instance, the electric motor will power the car for the pull away and up to a certain speed; usually a low 20km/h or so. It will then take over from the engine at a constant throttle cruise. By not using the engine in these situations, the vehicle as a whole is more efficient.

Performance hybrids use their electrical energy in a somewhat different way and these vehicles will usually allow the driver to select between an automatic, fully electric and sport mode. In auto, the computer will decide when it is most efficient to use electrical power while in full electric there is no fossil fuel used. The sport mode (as found on the BMW i8) uses both the petrol engine and the electric motor to propel the car and give it a combined output.

Most hybrid offerings will have a mode that will allow the motion of the vehicle and rotation of the wheels to generate an electric current that will charge the batteries. The Sport mode in BMW and the system in the Toyota hybrids for instance charge the batteries to a point while the petrol engine runs. This allows you to switch to full electric mode and run off of the power you generated while travelling, essentially giving you “free” propulsion. Used effectively with an understanding of the system in effect, a hybrid car is more efficient than its fossil fuel counterpart – and it's kinder to the environment.