Published in European Automotive Design 2008

Shocked by rising fuel prices, worried by a combination of uncertain oil supplies, changing climate and looming recession, car buyers are starting to think small. The biggest market changes are happening in the US, where the big three are ramping up production of I4 engines in response to buyer demand. At the same time, political pressure is building on OEMs to reduce the CO2 output of vehicles they build. So lately all the talk around the industry is about the phasing out of V12s and V8s in favour of turbocharged V6s and in-line fours, and the reduction of engine capacities across the board.

We’ve been here before, of course. Turbocharged small-capacity gasoline engines saw a rapid rise in popularity in the 1980s, driven by high fuel prices in the wake of the two major oil supply crises of the 1970s. Back then, forced induction was seen as a way of providing high-performance engines without the penalty of high fuel consumption. Now, pressure charging looks like the way forward to smaller, more efficient powertrains that can achieve more with less in all vehicle sectors – more power, or at least the same power, with lower fuel consumption, while meeting targets for lower CO2 emissions. Effective use of downsizing technologies – and a future plagued by fuel price spikes, which are almost inevitable – might just be persuading buyers to take smaller engines more seriously.

Larger SI engines are already being challenged by the latest generation of diesels, which have improved out of all recognition in recent years – thanks largely to the widespread adoption of common rail injection technology. But because SI engines are inherently low cost and low weight compared to diesels, they are still very competitive in small and medium sized cars. It’s here that downsizing approaches can make significant improvements to fuel economy and CO2 emissions.

Reducing the swept volume of an SI engine improves its efficiency in a number of ways. Pumping losses drop because the volume of air moved during each engine revolution is reduced. Smaller engines are also likely to run at higher loads in a given duty cycle, which implies wider throttle openings, itself leading to a reduction in pumping losses. The more compact combustion chamber of a smaller engine offers less internal surface area available for heat transfer and a shorter flame path, reducing heat losses and potentially improving cold-start performance. Frictional losses are minimized by smaller, lighter reciprocating components and perhaps by a reduction in the number of cylinders, and overall powertrain weight is reduced, to the benefit of vehicle performance.

Of course, an engine with a smaller swept volume pumping less air per revolution will also generate less power. At part load the driver will feel he has to work the engine harder for a given degree of vehicle performance, while at full load the power available might be considered inadequate. Higher engine speeds might be able to liberate more power, but only at the expense of extra noise, wear and frictional loss. Development of downsized engines is concentrating not just on producing smaller motors which can achieve high power outputs, but on delivering the performance characteristics and driver appeal of much larger power units. That means making the most of the available cylinder capacity at all engine speeds, maximising volumetric efficiency all through the engine speed range. This can mean extending existing technologies to new engine capacity classes and vehicle sectors: more and more engines will adopt four or five valves per cylinder, variable valve timing and lift, unthrottled operation using gasoline direct injection (GDI) and pressure charging.

It is pressure charging which is seen as the most important technology in downsizing, as it improves the potential for air throughput and gives a downsized engine the ability to produce high maximum power. The challenge is to implement pressure charging in such a way that a downsized engine provides apparently instant response in transient conditions at all engine speeds, just like a large, normally-aspirated engine.

So far, there seems little consensus on whether engine-driven superchargers or exhaust-gas turbochargers are the best way forward. Turbocharging promises high part load efficiency but lacks instant low-speed response. Supercharging provides the immediate response drivers want, but at the expense of extra noise and lower compressor efficiency at high engine speeds and pressure ratios, leading to a fuel consumption and CO2 emissions penalty.

One answer might be to mask the turbocharger’s tardy low-speed response using a mild hybrid drivetrain with a supplementary electric motor. This is the approach taken by Lotus Engineering and Continental with their ‘Low CO2’ Opel Astra concept, demonstrated in 2008. The engine is a three cylinder 1.5-litre unit developing 160ps. It features variable valve timing and lift (using Lotus’ Cam Profile Switching system) and a new cylinder head with an integrated exhaust manifold, which is said to deliver a 35% reduced parts count, a 20% weight reduction and improved durability. The combination of a downsized engine and 12kW electric motor gives the Astra demonstrator similar performance to a conventional 1.8-litre engined car, with a 15% improvement in CO2 emissions.

Volkswagen’s answer is to combine both pressure charging technologies in one powertrain, in an attempt to get the best of both worlds. Its TSI engines were introduced in 2006, and are probably the most high-profile downsized units to date. Beginning with the normally-aspirated EA111-series I4 engine, Volkswagen engineers designed a new grey cast iron cylinder block with high cylinder pressures in mind: the new component is said to resist cylinder pressures of 21.7bar for sustained periods. The existing FSI gasoline direct injection system was modified, with a new multiple-hole injector and the injection pressure raised to 150bar. A belt-driven Roots-type supercharger and conventional wastegate-controlled turbocharger were added, the supercharger driven from the engine coolant pump via a magnetic clutch. At low engine speeds (up to 2400rpm at low loads, 3500rpm at full load) the clutch is engaged to drive the supercharger. At higher speeds the clutch opens and the turbocharger takes over. The result is a 1.4-litre ‘twincharger’ engine with 200Nm of torque at just 1250rpm and up to 240Nm between 1500rpm and 4750rpm, and a 170ps maximum power output. Volkswagen equates the TSI’s performance to a conventional 2.3-litre normally aspirated engine, but with a 20 percent improvement in overall fuel consumption.

The combination of mechanically-driven supercharging and turbocharging, though apparently effective, is a complex solution which must be expensive to engineer and expensive to build. While smaller TSI engines are reportedly under development for A- and B-class vehicles in the Volkswagen family – which suggests that the cost issues can be overcome, or at least minimised – there are simpler and cheaper alternatives which look just as promising.

One low-cost approach has been proposed by Antonov, which has developed a two-speed centrifugal supercharger based on a Rotrex design. The supercharger is driven by a tiny automatic gearbox, which provides a 1.36:1 overdrive ratio at low engine speeds and a 1:1 ratio at high speeds. The higher gearing at low speeds runs the supercharger faster to improve torque, while the lower gearing used at high engine speeds avoids excessive power consumption by the supercharger at high speeds. Antonov claims the step in the torque curve which results at the gearchange point is not noticeable to the driver. The big advantage of the Antonov/Rotrex system is its low cost and autonomy: it is small, simple, easy to package and needs no external electronic or hydraulic controls.

A further step towards optimisation of the supercharger could be a continuously variable drive, which would probably be more costly than the two-speed Antonov drive but would potentially provide a greater benefit. One system is the SuperGen, which is being developed by Integral Powertrain and NexxtDrive. A 12V control motor is used to turn the planets in a Rotrex epicyclic traction drive, with the ring driven by the engine and the sun driving the compressor. Control of the electric motor varies the gearing to drive the compressor at anything from up to 150 times crankshaft speed. Throttle response at low engine speed is said to be comparable to a large normally-aspirated engine, and Integral Powertrain suggests longer intermediate gears can be used without compromising response. As a result, on a 1.4-litre engine in a C-segment vehicle the system is said to deliver a 20% reduction in CO2 compared to a conventional 2.0-litre normally aspirated powertrain. Integral Powertrain claims a downsized, SuperGen-equipped engine delivers a greater CO2 reduction than ‘twincharger’ technology, at similar cost.

An alternative is the electric supercharger. Controlled Power Technologies, which took over advanced powertrain technologies from Visteon and Emerson in 2007, released details of its Variable Torque Enhancement System (VTES) in 2008. VTES uses a ‘switched reluctance’ electric motor to drive a low-inertia centrifugal compressor which idles at 5000rpm and can accelerate to its 70,000rpm maximum speed in just 0.35 seconds to provide near-instant response during transient operation at low engine speeds, while a conventional fixed-geometry turbocharger takes over at the top end of the speed envelope. CPT claims the system performs just as well as its mechanical counterpart, but is more cost effective.

Whether turbocharged or supercharged, or even boosted by a combination of the two, previous generations of pressure charged engines have been designed with lower geometric compression ratios than their normally-aspirated counterparts, to avoid the onset of knock at high boost pressures and high intake charge temperatures. But lower compression ratios reduce thermal efficiency, and compromise low-speed response to throttle inputs. One of the key features of recent downsized engine concepts is the use of relatively high compression ratios – VW’s series production TSI engines run at 10:1, for instance, while the Lotus/Continental Low CO2 concept demonstrated in 2008 has a compression ratio of 10.2:1. Ricardo says its Lean Burn Direct Injection concept, a 1.1-litre GDI engine with variable geometry turbocharger, operates at 1.5 compression ratio points higher than a conventional pressure charged engine – implying around 10:1. Compression ratios this high are similar to those of current normally aspirated engines, never mind pressure charged units. In the past mixture enrichment has been used for its charge cooling effect, but the extra CO, CO2 and hydrocarbon emissions it generates makes it unacceptable for the future. Instead, downsized engines utilise direct injection, which causes the intake charge to cool as the liquid fuel vaporises in the combustion chamber.

Even higher compression ratios would benefit response of turbocharged engines at low engine speeds, but could only be contemplated if knock could be avoided at high boost pressures. One way to achieve that is to vary the compression ratio, and several methods have been developed, including eccentric main bearings, variable combustion chamber volumes and variable stroke systems, but none has yet reached production. It will be interesting to see if the potential benefits outweigh the mechanical complexity and cost inherent in all these systems. Variable valve operation with a late inlet valve closure (LIVC) strategy would be a simpler, if less efficient, alternative.

Whatever the chosen downsizing strategy, engineering the powertrain could turn out to be the easy part. Persuading buyers that a turbo four really can do the job of a V6 or V8 – and can offer similar levels of durability – might be a more challenging task. Markets like North America, where large-capacity normally aspirated engines have been the norm for as long as anyone can remember, will be particularly tough to crack. Even in Europe, with its historically higher fuel prices and greater devotion to the small, high-efficiency engine, the assumption has always been that a larger engine capacity means more power, more performance and higher status. Developing technology might soon mean that changes: turbocharged performance cars have already shown that there’s more to powertrain performance than simply swept volume. That might come as quite a shock to car buyers in some market sectors – and the industry will need to work hard to convince them that clever design is more important than mere size.

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Andrew Noakes motoring journalist / author / lecturer