Infiniti QX350 - Variable Compression Engine

Compression Ratio

Every piston engine designer since the first internal combustion engine has had to make a critical decision during their design process. The question – what compression ratio will this new engine have? The compression ratio denotes the size of the combustion chamber at its largest capacity, when the piston is in the lowest position or Bottom Dead Center (BDC). And, at its smallest capacity, when the piston is in its highest position or Top Dead Center (TDC). A standard compression ratio for consumer gasoline vehicles is 8:1. This means that at TDC, the size of the combustion chamber is 8 times smaller than at BDC.

So why is this choice important for designers? If an engine’s compression ratio is too low, it will be less thermally efficient and have lower fuel economy. This is due to the fact that higher compression ratios allow combustion to occur with less fuel. However, if the compression ratio is too high, engine knocking – also known as detonation – may occur. Engine knocking is caused by the air / fuel mixture igniting at any time other than the precise moment it is intended to during the piston’s stroke. This detonation can completely destroy the engine if knock sensors are not able to modify the ignition timing.

Variable Compression

However, INFINITI has revolutionized the piston engine with their new 4-cylinder variable compression turbo engine. This engine is able to modify the compression ratio that it operates at during engine operation. In fact, it can change from a ratio of 14:1 to 8:1 in under 1.5 seconds. This ratio is fully variable and can operate anywhere within this range of ratios. So, when the engine is running under low load such as at idle or highway speeds, the compression ratio will be modified to be higher for increased efficiency. However, under heavier loads the compression ratio will lower. The 2.0 liter engine will output 268hp when the turbo is fully spooled up and cramming as much air as possible into the combustion chamber with a low compression ratio.

The benefits of this new design don’t stop there, though. Previously, all 4-cylinder engines required balancing shafts to compensate for engine vibrations due to the crankshaft design. INFINITI’s new engine does not require any balancing shafts because of a new crankshaft design. This design is being used in conjunction with the actuator arm and other mechanical linkages required to adjust the compression ratio. The INFINITI engine is not only the first variable compression ratio engine, but also the first inline 4-cylinder that is inherently balanced at ALL compression ratios.

More Information

The first vehicle to utilize this new engine will be the INFINITI 2019 QX50. There’s a lot more ingenuity inside this engine that you can read about Here.