TECH ED’S CHOICE

FUEL EFFICIENCY AND COMPRESSION RATIO

I buy CAR to read about new concept cars and future technologies. What interests me is the downward trend in fuel consumption of new cars and the link to compression ratio. The little I know about increasing compression in an existing engine is to skim the cylinder head (or use a thin head gasket), or use longer con-rods. These are known methods of increasing performance but what fascinates me is how the fuel consumption is reduced at the same time. SIMONS IHEMBA Namibia

Fuel consumption is all about efficiency and achieving small gains in many areas. Taking a holistic approach on energy, you have to consider how much energy is needed to propel a vehicle and how efficiently the powertrain is in converting the chemical potential energy of fuel (or batteries in EVS) to a motive force. As we’re focusing only on technology concerning internal-combustion engines, we’ll remove driving style (a significant contributor to fuel consumption) from the equation.

The following important factors influence fuel consumption and, by targeting and reducing these, fuel usage can be trimmed: vehicle mass; rolling resistance of wheels; aerodynamic drag; and powertrain losses.

If we focus on the engine, there are many factors that would influence the efficiency, including: • Internal frictional losses (valvetrain,

piston movement, etc.); • Pumping losses, especially in petrol engines with a throttle. This is especially evident in large-capacity engines during part-load conditions; • Engine design parameters (example: the

combustion chamber shape); • Backpressure in the exhaust system; • Turbocharging, as it harnesses some of

the wasted exhaust energy; • Charge temperature. The cooler the intake charge, the better (therefore, intercoolers are used in turbo applications); • Optimal ignition timing in petrol engines

and injection timing in diesels; • Direct injection allowing for higher compression ratios in petrol engines thanks to charge cooling preventing knock (auto ignition); • Compression ratio (CR), as it influences thermal efficiency. Let’s focus on the compression ratio. Thermodynamically speaking, the higher an engine’s ratio, the higher the combustion efficiency. The reason for this is mostly the higher temperatures of combustion and increased expansion ratio during the downward stroke of the piston, allowing more useful energy to be extracted. Naturally aspirated engines rarely exceed 12,5:1 (Mazda’s Skyactiv engine at 14:1 is a special case) because of the onset of knock or auto ignition, where the fuel-air mixture explodes because of the elevated temperatures and pressures, and can cause terminal engine damage.

Diesel engines aren’t limited by knock because the combustion process actually requires high pressure and temperature to commence (no spark plugs) and is called compression ignition. The theoretical maximum compression ratio is therefore not restricted, but material properties limit the maximum temperatures and pressures sustainable; the highest ratio is around 23:1 in naturally aspirated diesels. The reason why modern turbodiesel engines employ a CR around 16:1 is because NO emissions dramatically increase with combustion temperature. There is also a drop-off in efficiency gain above 16:1.