Variable compression ratio

Gasoline engines have a limit on the maximum pressure during the compression stroke, after which the fuel/air mixture detonates rather than burns. To achieve higher power outputs at the same speed, more fuel must be burned and therefore more air is needed. To achieve this, turbochargers or superchargers are used to increase the inlet pressure. This would result in detonation of the fuel/air mixture unless the compression ratio was decreased, i.e. the volume above the piston made greater. This can be done to a greater or lesser extent with massive increases in power being possible. The down side of this is that under light loading, the engine can lack power and torque. The solution is to be able to vary the inlet pressure and adjust the compression ratio to suit. This gives the best of both worlds, a small efficient engine capable of great power on demand. In addition, VCR allows free use of different fuels besides petrol e.g. LPG or ethanol .

Cylinder displacement is altered by using a hydraulic system connected to the crankshaft, and adjusted according to the load and acceleration required.

Variable compression engines have existed for decades but only in laboratories for the purposes of studying combustion processes. These designs usually have a second adjustable piston set in the head opposing the working piston.

In 2018 Infiniti began production of their variable compression turbo engine, which uses a mechanical linkage to achieve the variability. It was installed in their QX50 SUV. The engine can produce any compression ratio from 8:1 to 14:1. The highest torque is achieved at 8:1, giving high acceleration, while the best gas mileage (fuel efficiency) is achieved at 14:1. The electronic engine controller responds to the pressure on the gas pedal, in real-time, altering the compression ratio seamlessly. Although this engine has a displacement of 2.0 L, and is an inline-four engine, it does not use balance shafts to eliminate the secondary vibrations. It is inherently balanced by the mechanical linkage.

Due to the comparative simplicity of cylinder head design (lacking intake valves) it is somewhat easier to implement in two-stroke engines. From the late 90s on up models which expand on this idea have been available, such as from Yamaha,[1] which dynamically vary the size of the combustion chamber. As of late (in the 2000s) this technology has seen some renewed interest, due it being able to burn a wide range of fuels (e.g. including alcohols) such as the Lotus Omnivore.[2][3]

A much earlier commercialized two-stroke engine, but very small (18 cc) and not powerful enough to be very successful, was the Lohmann engine produced in the early 1950s as a retrofit engine for bicycles . This engine had a one-piece cylinder head and sleeve, whose distance from the crankshaft was adjusted by a jackscrew operated by cables from a twist grip on the handlebar. Compression adjustment was essential to the operation of this engine because it used compression ignition of a fuel mixture which was introduced prior to the compression stroke and which therefore ignited whenever the compression brought it to a sufficient temperature. This meant that the compression needed would vary with air temperature, engine temperature, and fuel type: with too much compression the engine would suffer premature ignition and with too little it would fail to ignite at all. Thus the operator had to adjust the compression continually as operating conditions varied. The Lohmann engine was produced for only about five years because the control of compression (simultaneously with fuel flow) required considerable practice, and because even at optimal adjustment it provided no more power than a moderately fit rider could provide without assistance.

The first VCR engine built and tested was by Harry Ricardo in the 1920s. This work led to him devising the octane rating system that is still in use today. Many companies have been undertaking their own research into VCR Engines, including Saab, Nissan, Volvo, PSA/Peugeot-Citroën and Renault.[4] The 2019 Infiniti QX50 is available with a production version of the turbocharged variable compression engine.[5][6]

Peugeot MCE-5

Principle of the MCE-5 ("Multi Cycle Engine - 5 parameters"), the variable compression ratio engine made by Peugeot.

The Peugeot design works by varying the effective length of the con-rods connecting the piston to the crank. When the con-rod is shorter, the compression ratio is lower and vice versa. On the left hand-side of the diagram is the conventional piston of an internal combustion engine. On the right is an hydraulic cylinder with double-acting piston. This acts through a rod-crank system with a gear wheel, whose movement adjusts the effective con-rod length and thus the compression ratio in the left cylinder.[7]

• Continuously variable transmission
• DiesOtto
• Miller cycle
• Variable valve timing
• Infiniti VC-T - First Variable Compression Engine - Road & Track

• Waulis Patent Application
• Saab press release
• Automotive engineering international article
• Infiniti Introduces the First Production Engine With Variable Compression