Paul Horrell 09 October 2012

Naturally aspirated vs forced induction

Our resident expert referees the brawl under the bonnet

The Clio RenaultSport has just gone out of production, leaving the number of naturally aspirated full-on hot hatches at precisely zero. The BMW M3 and the old-shape-booted 118i and 120i, cars in the last year of their lives, are the only models in the entire BMW range not to have one or more turbochargers. The next M3, all 1-Series and the next Clio RS will get turbos. The overwhelming buzzword across the entire petrol-driven world is downsizing - reduce displacement, cut cylinders, add a turbo or two. Is the time coming when we'll read the obituary of the naturally aspirated petrol car engine? Must we resign ourselves to the loss of big revs, the loss of an instant and proportional answer to the throttle foot, the loss of the goading yell of an exhaust unencumbered by a turbine - things that only an unblown engine can give?

Well, hang on. Ferrari and Aston Martin, suppliers of the naturally aspirated F12 and One-77, would seem to disagree. You might think those two magical V12s prove exactly why forced induction will never win. Or you might mark them as epic last hurrahs of a dying breed.

We all know why the turbo is dominating. During official consumption tests, and indeed in everyday wafting around, smaller engines drink less than big ones. They're running in the more efficient part of their load band, and their frictional, thermodynamic and thermal losses are lower too. Crucially, consumption is proportional to CO2, since petrol (compounds of carbon and hydrogen) burns and is catalysed into CO2 and H2O. Both the cost of fuel and low-CO2 tax incentives are only pushing buyers one way.

But what good is that if the resulting caris so slow it can't get out of its own way? You need a turbo bolted to the side of your small-displacement engine. Having sat there quietly minding its own business during gentle running, it charges to the rescue when you floor the accelerator. Propelled by a turbine in the exhaust stream, it compresses the air coming into the engine. More air molecules can be fitted into each cylinder, and burn proportionally more petrol (but hey, it's outside the regime of the official test). The small-displacement Bruce Wayne transforms into a big-block Batman, burning more fuel to produce more power when you need it, reverting to small and economical when you don't. All's well.

Except that in place of the sound and instant response of a multi-cylinder naturally aspirated engine, you get lag and a dull drone. And if you drive a small turbo engine hard, your consumption may be no better than a bigger n/a engine. But small turbo engines do have other advantages. They're lighter and more compact, from which should follow better packaging and handling. Besides, they're cheaper to make than multi-cylinder ones, some of which saving a manufacturer ought to pass on to us.

While turbos seem to be winning the war, over the decades, there have been strings of little individual battles, where turbo and n/a engines have punched and counter-punched technical innovations to overcome their weaknesses.

Early turbos were mostly a route to power in the absence of any available or affordable alternative. Chevrolet launched the turbo Corvair Spyder in 1962, and then Porsche the 911 Turbo in 1974. Both had flat-sixes in the rear, with no room for anything physically bigger. And for Saab in 1978, not having the wherewithal for six cylinders, turbocharging the existing four was a handy shortcut. Saab made a better job of it than BMW's highly strung 2002 Turbo (Europe's first petrol turbo in 1972), but, even so, ‘off-boost lethargy' and ‘turbo lag' became staple phrases whenever these engines were being talked about. A naturally aspirated engine gives the full possible torque for a given rpm as soon as you ask for it. It might not be as strong as the torque from a turbo engine after the lag's passed, but a bird in the hand, and all that...

To try to get near the peak torque of the turbos, the natural-aspiration team fought back with some clever ruses. With its VR6, VW simply jammed a big engine into the space of a small four by wedging in two extra cylinders. Simple variable-cam phasing is widespread now, and it varies inlet/exhaust overlap to suit high-rev power, mid-rev torque and low-rev emissions. Honda's VTEC system and Rover's VVC were among the first to use differing cam profiles at different revs, giving an optimal timing and lift profile for mid-rev torque, and another for high-rev power. BMW's Valvetronic and Fiat's MultiAir are even more sophisticated ways of controlling valve timing and lift, plus they let the engine run without the throttle and its associated pumping losses. Naturally aspirated engines also often have variable intake tracts, to introduce various resonant lengths to charge the cylinders more effectively at various revs.

Well, if n/a engines were going to get themselves more torque, squashing lag was a priority for turbos. Anti-lag systems for rally engines such as the Mitsubishi Evo injected air and fuel into the exhaust when the throttle was shut; it caught fire in the heat, and the explosions kept the turbo spinning. Magnificently incendiary, crazily wasteful, and destructive and dirty - not exactly acceptable on the road.

On road engines, smaller turbos help: they have less inertia. But they don't work as well as bigger ones for high-speed power, so some engines use pairs of blowers in sequence. Or twin-scroll turbos, which separate the exhaust tracts of the cylinders that would otherwise wastefully interfere. A lateral-thinking solution is to use a small supercharger for low-rev pick-up. But superchargers use energy at high revs, so VW's Twincharged 1.4 and the new Jaguar C-X75 engine bypass and declutch the supercharger at high revs and hand off boosting duties to a turbo. Hmm, complicated. More common now are variable-geometry turbos. They change the angle of the vanes that guide exhaust across the turbine, so they're efficient at a wide range of exhaust flow rates. They were used in diesels for yonks, but their mechanisms were prone to fail in the higher heat of petrols. Beginning with the Porsche 997 Turbo, new materials have resolved that.

Handily, as naturally aspirated engines chased torque, as a good rule of thumb along came improved efficiency. Direct fuel injection (DI) adds more. It means an engine can run higher compression without knock, because the fuel is injected just before spark. Through clever exhaust and piston design, Mazda's new DI SkyActiv engines get it to 14:1. And compression equals efficiency. When it's combined with full variable-valve control, as BMW did in its last pre-turbo fours and sixes, you had an engine that was the poster child of n/a economy and civilisation.
Thing is, almost every technology that works well on n/a engines works even better on turbocharged ones. If knock matters on n/a engines, it matters more on boosted ones, which have, in effect, higher compression ratios once boost arrives. So DI is even more handy to have. Same for variable-valve control. All its benefits on n/a engines are redoubled on turbos.

Strangely, one of the final places we'll see n/a engines is in hybrids. In mild hybrids, the electric motor doesn't only help efficiency, it helps fill the low-rev torque hole. Ferrari's next Enzo will exploit that double win. In full hybrids, whether the Toyota system or in range-extenders like the Ampera, the hybrid system allows the petrol engine to run only around its most efficient middle rpm range. But that's the opposite reason to why we love n/a engines. We love them for their revs and sound.

Turbos tend not to rev because they don't need to: big on-boost torque allows higher gearing. Anyway, DI turbos are hampered because direct-injection systems become hugely expensive if they have to work above about 6,500rpm.

Turbos give us more performance and more economy. So you're left asking which of an/a engine's particular delights has the turboyet to give us. Is there no substitute for revs? The Jaguar C-X75 500bhp turbo motor runs to 10,000rpm. Or if you want instant torque, is there no replacement for displacement? A Viper buyer might say so, but AMG's turbos hardly feel limp-wristed. And in America, Ford's V6 EcoBoost-powered F150 pickup is beginning to convince the good ol' vee-eight's most loyal followers that there may be merits to this new-fangled itty-bitty 3.5-litre.

But we'll regret the passing of the sound and the instantaneous response of a good unblown engine. Sure, lag has largely gone now - it's more a slight softness in the pedal - but without that softness, with the hard-edged bite of a good n/a engine, you're harder-wired into the experience.

Finally, the noise: try the new M135i, and you'll be convinced BMW's straight-six petrol turbo is a fine sonic replacement for a heavier n/a V8, but the trouble is not many people are building straight-sixes. Even BMW petrols are mostly fours these days. At least Ford is binning its boring 1.6 n/a fours in favour of a charismatic little blown triple, and it won't be the only one. Generally, though, we're suffering not just because turbos mute the exhaust, but because downsizing means the dull-sounding blown four-cylinder is taking over from fives and sixes.

Maybe the price for the turbo engine's performance, economy and lightness boils down to this: we're going to have to live with engine noise that's electronically and synthetically enhanced, rather than the real thing. That's not so far-fetched, either. It's coming on this autumn's new Mondeo.

Words: Paul Horrell 

This feature first appeared in the September 2012 issue of Top Gear magazine

 

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