Farmer's Weekly (South Africa)
Petrol engine mysteries unravelled
Jake Venter is often asked questions on technical aspects of motoring. Here are a number of interesting queries that crop up regularly.
How should I drive to reduce engine wear?
An engine is like a Border collie; it likes to work. Cruising at a driving speed where the engine speed is 50% to 70% of the maximum shown on the rev counter causes the least amount of wear. Running below normal operating temperature, such as during short trips, accelerating hard, and allowing the engine to idle for long periods, causes the maximum amount of wear.
Why is it harmful to drive a car when the engine temperature is below the normal operating temperature of 90°C–100°C?
A cold engine needs to be supplied with a richer fuel-air mixture to run smoothly. This results in a quantity of unburnt fuel, some of which ends up in the sump, where it degrades the oil. This means that last week’s short trip has degraded the oil for this week’s motoring. If short trips cannot be avoided, the oil should be changed more often.
What’s the difference between torque and power?
Torque is the rotational equivalent of a force. This means that the force developed by a rotating engine is measured in torque units (N.m). A car’s pulling power up a hill, or when towing, depends on the torque available.
Power is the rate of doing work; it is calculated by multiplying available torque by engine revolutions per minute, then dividing by a particular constant to get an answer in kilowatts (kW). A car’s maximum speed and its acceleration times to a chosen speed depend on the power output.
Why has pressure-charging an engine become so popular that some manufacturers no longer make naturally aspirated engines?
Most manufacturers favour the turbocharger. This is an air pump driven by the exhaust gas; it forces more air into an engine than it would normally be able to suck in. More fuel can be supplied so that the engine becomes more powerful.
For any given power output, a turbocharged engine will be physically smaller and weigh less than a naturally aspirated engine. This factor on its own will improve fuel consumption but there is an additional benefit. The turbo is driven by energy in the exhaust gas that would otherwise be wasted. Such engines are therefore more fuelefficient at part-throttle operation.
A few manufacturers fit a supercharger. This is driven by the engine’s crankshaft and therefore robs energy from the engine. Its use results in higher low-down torque delivery, but it cannot deliver the fuel economy benefits of a turbo.
How effective has engine downsizing really been?
Downsizing is the process of reducing engine capacity and/or the number of cylinders but keeping power output almost unchanged by using a turbocharger. It is really effective only for larger engines, which can cruise at a reasonable speed without using large throttle openings.
Smaller engines, especially one-litre turbocharged units, have in general proved to be fuel-thirsty. The engine control unit (ECU) normally keeps the fuel-air mixture close to chemically correct to assist the catalytic converter. But when large throttle openings are used on a turbocharged engine, the ECU enriches the mixture so that the evaporation of the extra fuel lowers the combustion temperature to avoid harmful detonation.
Why is a hybrid – a car with both an internal combustion and an electric engine – more efficient than a one-engined car?
A hybrid employs a smaller petrol engine than normal for a given body size to save fuel, and then augments the power output when required by bringing an electric motor into action. The latter becomes an alternator during braking or coasting to capture the car’s kinetic energy and use it to charge the propulsion battery. The battery is also charged by the engine when necessary so that a hybrid never needs mains recharging. Some hybrids have a larger propulsion battery and plug-in fittings, enabling them to be charged at home and used like a plug-in electric car.
• Jake Venter is a journalist and a retired engineer and mathematician. Email him at jacobventer77@gmail.com. Subject line: Auto Engineering.