Different strokes

08 February 2010 | By Jon Excell

Two-stroke engines are dirty and noisy; that’s the common perception at least. But according to engineers at Lotus, a new breed of ultra-efficient two-stroke, a world removed from the puttering, dribbling power-train of the low-cost moped, may one day steal the four-stroke’s thunder.

It’s early days, but the Norfolk-based firm’s prototype Omnivore engine, a direct-injection two-stroke fitted with a variable-compression ratio system, certainly mounts a compelling case. Initial tests on a single-cylinder test-bed engine —; optimised to run on alcohol or gasoline-based fuels —; showed a 10 per cent improvement in fuel consumption compared with today’s most efficient automotive engines.

It sounds like a pretty radical departure. But according to Jamie Turner, chief engineer of powertrain research at Lotus, if it wasn’t for a few quirks of history, we might all be driving around in two-stroke powered cars today. ’If you look back years and years ago, it was easier for people to understand the four-stroke cycle than the two-stroke cycle. If you imagine you’re an engineer at the dawn of a new age and there are various technologies lined up in front of you, the four-stroke cycle — where everything happens sequentially — was easier to get your head around than one like the two-stroke cycle, where the process of inducing charge, combusting it and exhausting it had various elements happening simultaneously.’

Turner added that while the four stroke may be the dominant source of power in the car industry, two-stroke engines actually cover a wider range of uses and are fundamentally more fuel-efficient.

The reason for this is that four-stroke engines suffer greatly from efficiency reductions associated with throttling losses. ’When you drive a four-stroke engine normally around town you don’t operate it at full throttle and the engine still has to do work to pull air past the throttle. The throttling loss goes up as you reduce the load of a four-stroke engine.’

As engine designers continue to nibble away at inherent inefficiencies, throttling loss is becoming a more important issue. Indeed, most of today’s big powertrain developments —; from downsizing by developing small, highly boosted engines, to stratified approaches that open the throttle and manage the burn more effectively —; are all, essentially, different ways of offsetting throttling loss.

But another approach to this problem, said Turner, is to use a two-stroke engine. ’Two stroke doesn’t do its gas exchange in the same way and doesn’t suffer from throttling loss like the four stroke.’

Eliminating this throttling loss was the starting point for the Lotus Omnivore, but one of the most significant features of its design is the use of a variable compression ratio (VCR) system, which improves the efficiency of fuel conversion by burning the fuel at a higher compression ratio.
Despite the huge promise of VCR technology, the mechanical complexity of integrating it with a four-stroke engine has hampered its commercial prospects; but, said Turner, there’s a far more natural fit with two-stroke engines. ’The commonest type of two stroke is one where you have ports in the walls to do the gas exchange and that frees up the cylinder head (in the four-stroke engine you have valves in the cylinder head). That means there’s a big area of the engine available for you to do something with.’

Turning his attention to the perception of two strokes as dirty and polluting engines, Turner pointed to many developments on the Omnivore that have addressed these concerns. ’There are many different types of two-stroke engine and the types people see on motorbikes are pretty simple devices where the exhaust and transfer ports are open. You get fuel going straight through and it’s filthy, but this is relatively easily fixed, using direct injection and not injecting until after the exhaust port closes.’

Turner’s team also endowed the engine with a far smoother torque curve than typically associated with two-stroke engines. It did this by using a novel charge-trapping valve that sits in the exhaust and goes up and down at engine speed and allows the exhaust timing to be changed.
But the other major feature of the engine is the combustion process, a derivative of homogenous charge-compression ignition (HCCI), where the engine operates without the need for the spark plug to ignite the fuel and air mixture in the cylinder. The process could bring many of the benefits of the compression ignition used in diesel engines but without the high NOx emissions. Turner explained: ’You have a mixture of air and fuel inside the engine and you squash it until it goes bang; by doing that, you can burn a lot of air with the fuel in the engine, not like a spark-ignition process where you have to have a very tightly controlled ratio. By having a lot of air there you cool the combustion down and reduce NOx emissions and fuel-conversion efficiency goes up.’

’Put all these things together and you have an engine that can burn all the fuel inside it very efficiently and then have very few losses associated with doing so,’ he added. ’All of those things give you quite a radically different type of engine that doesn’t actually use any technology that’s not [manufactureable] —; but it takes you to a different place.’

The initial programme was carried out in collaboration with Queen’s University Belfast and Orbital Corporation Australia. Lotus is now looking for further backing to share the risk and to take the project to the next level, which would involve developing a multi-cylinder variant and ultimately fitting a test engine to a car. So far, said Turner, the concept has certainly generated plenty of excitement. ’We’ve had a lot of interest from OEMs, which is good because everyone’s grappling with how to get CO2 down in the future so we’re hoping to form some kind of consortium that will enable us to fully map the engine and take the engine design forward. I believe the engine has significant legs on it.’
There’s a long way to go before the engine makes it anywhere near a car — Turner estimates three years with a favourable wind — but considering how fundamentally different it is, the project has already come a long way.

Recalling the light-bulb moment when a colleague suggested putting a VCR system in a two-stroke engine, Turner believes the new engine is a great example of the importance of a creative, cerebral approach to engineering. He finished by saying: ’The overall process was one of invention rather than computation. You would not have arrived at this solution through just running endless computer simulations, because pretty much all of the tools that the industry uses are based around four-stroke engines.’