Why the latest engine from Nissan can be a game-changer
Compression ratio is used to determine the efficiency of an internal combustion engine, where an air-fuel mixture is ignited to get things running. It’s the ratio of the volume inside the combustion chamber when the piston is at its lowest point to the volume when the piston reaches the highest point. In spark ignition (petrol) engines, it is at the latter point where the mixture is ignited by the spark plug.
A high ratio allows for better efficiency because it can squeeze more mechanical work out of a fixed mass of air-fuel mixture. During the expansion stroke, the exhaust gases are allowed to cool down before being expelled into the atmosphere, resulting in less energy loss. The downside, however, is the possibility of knocking—where the combustion process doesn’t synchronise with the timing of the spark ignition. This is why the compression ratio has to be just right.
Infiniti, Nissan’s luxury arm, has attempted to achieve this balance by constantly tuning this figure through a variable compression ratio (VCR) engine called the VCT. The four-cylinder turbopetrol boasts a compression ratio between 8:1 and 14:1. In a regular engine, the piston is connected to the crankshaft via a connecting rod, but in the VCT, the angle between them is altered using a Harmonic Drive, a type of gear which operates a control arm. This varies the maximum height the piston can reach during compression, resulting in variable ratios. Though Infiniti will be showcasing this technology at the 2016 Paris Motor Show, due to the sophisticated electronics and software involved, it is bound to be expensive.