In a conventional piston engine, the compression ratio defines the relationship between the largest and smallest volume of a cylinder - when its piston is at bottom dead centre and top dead centre respectively.
When it comes to designing or building a new engine, it's imperative that a suitable compression ratio is chosen. High compression ratios maximise efficiency and power output but also increase the chance of pre-ignition or detonation - particularly in force-fed applications - which can cause catastrophic damage.
A lower compression ratio reduces the chance of these issues but also decreases the efficiency and output of the engine - the latter of which can be compensated for through the use of forced induction. Designers of turbocharged or supercharged engines will consequently opt for a suitable, compromised compression ratio that permits adequate power and efficiency without inducing reliability concerns.
If the compression ratio in a force-fed engine could be varied, however, then the optimum ratio could be selected for the load which the engine is experiencing at that particular moment - allowing for its power output or efficiency to be maximised safely. Such a system is used by the Infiniti QX50 and its 'Variable Compression Turbo' engine.
How does variable compression ratio technology work?
The aim of variable compression is to deliver an engine that is efficient yet capable of producing a substantial amount of power. As a result, it is generally applied to downsized, force-fed engines. These smaller engines are inherently more efficient than bigger ones, while the use of forced induction permits them to produce power and torque equivalent to larger engines.
Getting the best from these downsized powerplants is easiest when the compression ratio can be varied, in order to prevent combustion issues under heavy load while also permitting improved efficiency when cruising.
A Focus ST, for example, has a compression ratio of 9.1:1. An engine with a variable compression ratio, on the other hand, could drop as low as 8.0:1 when accelerating - permitting plenty of boost and producing lots of power. When cruising and requiring little boost, that compression ratio could safely rise to 14:1 in order to maximise the engine's thermal efficiency and improve its fuel consumption.
Adjusting the compression ratio of an engine on the fly requires complex mechanical and electronic control systems, as well as a bespoke engine design that permits for variations in its geometry. One approach is to change the position of the cylinder head itself; the prototype 'Saab Variable Compression' engine, first unveiled in 2000, used this method. This supercharged 1.6-litre, five-cylinder engine featured a 'monohead' - a head with integral cylinders - that could be tilted by up to four degrees.
When the angle of the head was adjusted, in relation to the pistons and crank, it would cause the volume of the combustion chamber to increase or decrease - causing a respective change in the compression ratio. Saab stated that the operating range was from 8:1 to 14:1 and, at its peak, that the engine would produce 225hp and 224lb ft.
The modern Infiniti VC-T engine, which is the first production engine with variable compression, took a different tack. Unlike Saab, the Infiniti engine features a multi-link system that supersedes the conventional crankshaft and connecting rod configuration. Altering the angle of this linkage changes the maximum height of the pistons in the bore, increasing or decreasing the compression ratio as desired.
Are there any other benefits to variable compression ratio tech?
There are other reasons that a manufacturer may want to adopt a variable compression ratio system. High-compression engines experience high in-cylinder temperatures, for one thing, due to the increased compression - which results in excess nitrogen oxide emissions and possible cooling issues. An engine with a low compression ratio isn't as thermally efficient but produces less NOx, while its warmer exhaust gases allow catalytic converters to start functioning from cold quicker.
A variable compression ratio system could consequently allow a manufacturer to deliver the best balance of efficiency, output and emissions across a wide range of engine loads. There are a plethora of considerations, though, and the cost and complication of a variable compression system can often outweigh the benefits.
A brief history of automotive variable compression ratio technology
Saab was the first manufacturer to introduce the concept of variable compression ratio technology to the automotive market, introducing its prototype 'Saab Variable Compression' engine in 2000. Besides producing a substantial 150hp and 147lb ft per litre, the SVC was also claimed to offer a 30 per cent reduction in fuel consumption compared to a conventional naturally aspirated 1.6-litre engine.
Its emissions were also claimed to be capable of meeting future regulations. The project was abandoned not long after, unfortunately, reputedly due to cost-cutting measures from new parent company GM.
Other companies, including Peugeot, also explored the potential of using engines with a variable compression ratio - but it was Nissan's premium arm, Infiniti, which produced the first production engine in which the compression ratio could be varied. Its 'Variable Compression Turbo' engine was unveiled in late 2016 and, like the Saab, offered a compression ratio that ranged from 8:1 to 14:1. This engine ultimately made it into production in the 2018 Infiniti QX50.
The turbocharged 2.0-litre, four-cylinder engine produced a substantial 268hp and 288lb ft and, much like Saab's engine, it was mooted to offer a 27 per cent improvement in efficiency compared to a similarly powerful V6.
Small prototype engines with variable compression systems, however, have existed since the 1920s. These engines have been used to study combustion processes, fuel quality and engine design, and allow for quick acquisition of data across a range of compression ratios.