Wednesday, 22 October 2014
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Automotive, Rail and Marine

Omnivore

Partners: Lotus; Jaguar Land Rover; Queen’s University Belfast; Bioethanol and Orbital Corporation

omni

The Omnivore two-stroke engine can run on a variety of fuels

Given the excitement around electric and hydrogen-fuelled vehicles, it’s sometimes tempting to believe that the internal combustion (IC) engine has reached the end of the road. That  if engineers want to do more than nibble at inefficiencies they’ll need to look to more exotic technologies for the powertrains of the future.

Omnivore, a prototype two stroke engine that boasts 30 per cent fuel efficiency savings over many of today’s four stroke engines is a compelling piece of evidence to the contrary.

Able to run on a variety of fuels and boasting a number of major technical innovations, the engine is the result of a collaboration between Lotus Engineering, Queens University Belfast, Jaguar Land Rover and Australian fuel injection specialist Orbital Corp. 

Thanks to its lingering association with dirty, puttering moped engines, the idea of the two-stroke as a low-emissions saviour, is somewhat surprising. But according to  Jamie Turner, chief engineer of powertrain research at Lotus, two-strokes actually cover a wider range of uses and are fundamentally more fuel-efficient than four strokes.

This is because they don’t suffer from throttling loss: a major cause of efficiency reduction in four stroke engines. Turner explained: ‘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.’

Another advantage of the two-stroke is that its enabled the Omnivore team to make use of a variable compression ratio (VCR) system, which improves the efficiency of fuel conversion by burning the fuel at a higher compression ratio.
 

According to Turner, the difficulty of integrating VCR technology with four-stroke engines has hampered its uptake. But there’s a far more natural fit with two-strokes. ‘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.’

Another striking feature of the engine is its use of  homogenous charge-compression ignition (HCCI), which enables it to be started without a spark plug. ‘You have a mixture of air and fuel inside the engine and you squash it until it goes bang,’ explained Turner.  ‘We can crank and start immediately in HCCI with the ignition system removed from the engine,’ he continued.  ‘We do that by increasing the compression ratio, cranking the engine and just switching the fuel on.’

The group has already performed ambient cold starts using this system at -25 ° and hopes to be able to push this further with the next version of the engine.  ‘If we want to start at -30°C - which is the industry standard for cold start without assistance - we would need about 50:1 compression ratio,’ said Turner.  ‘That’s achievable once we get rid of the spark plug bowl. The next version of the engine will be very different and have a wider range of compression ratio adjustment and it’s entirely possible that you could have full range of operation without any need for a spark ignition system,’ he added.

The results speak for themselves.  ‘At the low speed low load range  - which is typical of the drive cycle  the we’re about 10 - 15 per cent better than what one would consider to be state of the art stratified four stroke combustion,’ claimed Turner.  ‘In itself that is significantly better than normal four stroke combustion to the tune of 10 - 15 per cent . So we would be looking at about 30 per cent improvement in fuel economy over a naturally aspirated engine.’

Achieving the same kind of reduction with a four stroke engine would, said Turner, be considerably more complicated and expensive.  ‘To achieve the same with a four stroke engine you’ve got to go to massively aggressive downsizing  and when you do this you need a degree of something to help you with the drivability at the bottom end. That can mean a degree of hybridisation, and when you start to chuck that in the costs go up a lot. What we’re proposing here is a relatively large displacement two stroke engine for which drivability will not be an issue. We think, with the right size  of engine - you could make this for the same amount of fuel consumption reduction on the drive cycle.’

graph

Test results indicate that Omnivore's fuel economy is about 15 per cent better than today's best four-strokes

Collaboration was, explained Turner absolutely key to the project’s success and Omnivore brought together a fascinating cross-section of expertise: from the academic two-stroke expertise of Queens University, Belfast to the broad industry overview of Jaguar Land Rover.

Another key technical partners was Australian fuel injection specialist Orbital Corp , which developed a spray guided direct injection system for the engine.

An evolution of the firm’s existing FlexDI technology which is widely used in marine and two stroke scooter engines, the system is able to directly inject gaseous and liquid fuels, or indeed a mixture of the two.  While the technology was  key to enabling the Omnivore to operate on a range of fuels, Orbital’s Simon Brewster explained that the project has also helped his firm improve their own offering. ‘Omnivore’s been extremely helpful,’ he said, ‘it’s always good to be involved with a company like Lotus. We’re looking at a wide range of programs around the world for the direct injection of alternate fuels. The ability to direct inject a liquid or a gaseous fuel through the same fuel system will become increasingly valuable - it’s a growth market.’

Omnivore remains a work in progress, and Turner’s team is continuing to refine and test the engine whilst looking for funding for a follow on project. The next step, he said, is develop a self-contained multi-cylinder test-bed engine and ultimately fit the engine to a vehicle.

The reaction from the wider automotive industry would suggest that this will happen sooner rather than later. ‘I know [Jaguar Land Rover] are very impressed with the results , but then again every OEM we’ve shown it to has been,’ said Turner. ‘Everyone’s grappling with the issue of 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.’

In the meantime this elegant rethink on the humble two-stroke engine continues to surprise and delight its inventors: ‘Every we time we do a piece of work there’s something else that astounds us’ added Turner. 

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