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Engine downsizing: Increasing efficiency and performance

For fans of large, naturally-aspirated engines, the writing has been on the wall for a while. As a society we have become more aware of our effect on Mother Nature, so we have been increasingly moving towards a greener future. Just what that means is yet to be clearly defined, with myriad opinions on the best way to move forward, however there is one area where everyone appears to agree: We need to reduce our reliance upon finite, ‘dirty’ resources to fuel our energy consumption.

This has not escaped the attention of the automobile manufacturers, who have reacted with massive investment in researching alternative fuels while simultaneously refining their production of petrol engines.

Some of that investment has borne production fruit- perhaps best embodied by the Toyota Prius hybrid. Others, like Nissan with their LEAF, have moved towards fully electric drive systems. Just hope you don’t run out of juice between charge points. Even supercars are getting in on the act, with Porsche introducing a hybrid system to their upcoming 918 Spyder.

While a complete market shift to electric power still many years off, the petrol-fuelled internal combustion engine is living under a stay of execution. They live on under ever-tightening regulations which demand reduced emissions (and therefore fuel consumption). Presently, Australia follows the standard-setting ‘Euro’ emissions standards, and is phasing in core ‘Euro 5’ compliance for passenger vehicles from late-2013, with full Euro 5 compliance expected in 2016. (For a detailed explanation of Euro 5 requirements, click here).

Despite these stringent measures it appears that we are in the midst of a power race amongst the higher-end manufacturers, with 400-plus kilowatts being the entry price for a ‘fast’ saloon, such as the BMW M5 or Mercedes-Benz AMG E 63. They make this power with engines of smaller capacity than their predecessors, but the benefit of turbocharging.  For reference, below are the pertinent figures for BMW’s previous and current M5s:

2004 BMW M5

BMW M5 V10

4999cc V10, 373kW at 7750rpm, 520Nm at 6100rpm, 14.8 litres per 100 kilometres

2013 BMW M5

2013 BMW M5 V8 Turbo

4395cc turbocharged V8, 412kW at 5750rpm, 679Nm from 1500rpm, 9.9 litres per 100 kilometres

As you can see, BMW traded revs (and instant throttle response, and sound- but I digress) for a chunk of extra power and masses of torque. This makes the new M5 far more liveable in day-to-day driving, and far more efficient to boot, with a 34 per cent reduction in fuel use.

“Surely turbocharging a car makes it less fuel efficient?” is a question often heard, the perception being that its larger power capability must come at the expense of fuel consumption. Simply, turbos use the engine’s waste exhaust gases to spin a turbine, which is linked via a shaft to a compressor ‘wheel’. This compressor ‘forces’ more air and fuel into the engine, creating what’s known as ‘boost’. The theory is this maximises fuel efficiency as well as improving power and torque, thanks to a more complete combustion process. And, as you can see from the above BMW data, the turbo motor doesn’t need stratospheric revs and big capacity to produce big numbers…although I do wonder if anyone could replicate the claimed fuel consumption!

As ever, the trend ‘at the top’ is being replicated lower down the automotive food chain. In 2012 Ford took the unprecedented step of introducing a turbocharged 2.0-litre four-pot to the Falcon, and it’s a great car to drive while being far less thirsty than the 4.0-litre naturally-aspirated six it sits beside. Renault’s up-coming Clio Sport will downsize from 2.0-litre ‘atmo’ four to 1.6-litre turbo four, with the same power output, more torque, and lower emissions.

Being a driving enthusiast, I miss the character of a big, revvy naturally-aspirated motor. But when it comes down to a choice between driving a smaller turbo-petrol or having to go all-electric, I’ll happily take the turbo…for now.

What are your thoughts on alternative fuels, engine downsizing and turbocharging for efficiency? Let us know in the comments.

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