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Infiniti's new VC-T engine explained

Infiniti's new VC-T engine explained
Omkar Thakur Omkar Thakur Wednesday 17 August 2016, 14:48 PM

Infiniti has built a new 2.0-litre turbocharged petrol engine for their upcoming crossover and it is already being touted as the next big thing in engine technology after the VTEC. We go through the patent images to find out how the new variable compression technology works.

EcoBoost series of engines from Ford sparked a barrage of small turbocharged petrol engines that replaced larger naturally aspirated mills. The reduced displacement promised better emissions to match the stringent norms and also better fuel economy. While the engines stood their ground on emissions, the real world fuel economy of the small turbo-petrol engines was not up to the mark.

As we all know, turbocharged engines run on a lower compression ratio to factor in for the higher pressure in the combustion chamber due to forced induction when the turbo spools up. This may result in premature detonation of the fuel charge resulting in loss of efficiency and power and also may stress the engine internals to failure.

But, when the turbocharger is yet to spool up (which is generally near the 2000rpm range), the engine is running low compression. In city traffic, cars do not really rev high and are caught in turbo lag more often than not. The engine, hence, guzzles more fuel every time you step on the throttle to compensate for the lower compression.

The Infiniti VC-T claims to run variable compression – high compression when the engine is under load through turbo lag and low compression when the turbo spools up. While that seems easy on paper, building a variable compression engine to deal with the thermal and mechanical stresses is a different ball game altogether.

Infiniti has devised an ingenious method evident from its patent drawings. The engine runs on variable crank radius which helps shift the top dead centre of the piston towards or away from the cylinder head, thus, varying the compression ratio. To achieve the required shift, instead of assembling the connecting rod on the crank pin, Infiniti engineers have pivoted a bell crank linkage on the crank pin to one end of which the connecting rod is fixed. The crank pin acts as fulcrum and the position of the link is monitored by a control shaft attached to the other end. The control shaft is controlled by the actuator arm which is electronically operated by the ECU.

When the engine starts, it runs high compression (14:1) with the actuator stretched out – running like a normal naturally aspirated engine. As the turbo spools up and starts forcing air into the combustion chamber, the ECU pulls the actuator arm in lowering the compression ratio in the combustion chamber to as low as 8:1. This helps in achieving minimal fuel consumption in both cases and also ensures complete combustion and low emissions in all scenarios.

The 2.0-litre VC-T develops 268bhp of power and 390Nm of torque. While it is not the highest in terms of power output for similar-spec engines, the torque output is at par with the leaders. It also claims to offer a better torque curve and 27 per cent better fuel economy overall. What it also means is the engine construction will become more complicated, more dependent on electronics and sensors and with more mechanical linkages may incur higher friction losses. We will have to wait till this technology goes into production to see if it works in real world, the chances of which seem bright as of now.


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