• Dec 10, 2009
Last March at the Geneva Motor Show, Lotus announced its new Omnivore research engine. The Omnivore is a two-stroke direct-injected engine designed to take advantage of the latest in electronic engine management and run on just about any liquid fuel (hence the name). In the time since the initial announcement, the boffins at Lotus Engineering have been exercising their creation on the dyno stand to evaluate its performance.
The single-cylinder engine uses an air-assisted direct injection system. A movable "puck" in the top of the cylinder head allows the compression ratio to be varied.

The engine has so far been run on gasoline in both spark ignition and homogeneous charge compression ignition (HCCI) modes. The HCCI mode is of particular interest because it is capable of providing diesel engine-like efficiency without the particulate and NOx emissions that require expensive after-treatment systems in a standard diesel engine. Lotus is claiming the Omnivore can operate in HCCI mode in a wide variety of operating conditions and even from a cold start, something that has been problematic for previous HCCI engines.

According to the initial test results, the Omnivore is achieving up to a 10 percent improvement in efficiency (as measured by Indicated Specific Fuel Consumption) compared to existing spark ignition direct-injected engines.

There is clearly a lot of work yet to do, but the concept behind Omnivore shows significant promise.



[Source: Lotus]

press release

Lotus Omnivore Engine – 10% Better Fuel Economy than Current Leading Gasoline Engines

Initial phase of Omnivore development achieves 10% improvement in fuel consumption compared to stratified direct injection engines, also with ultra low emissions. The research signals a potential paradigm shift with engine 'upsizing' for increased fuel economy.

The first testing phase of Lotus Engineering's Omnivore variable compression ratio, flex-fuel direct injection two-stroke engine has been successfully completed on gasoline. In addition to exceptional fuel consumption results, the engine has successfully demonstrated 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 - down to extremely light loads. Traditionally, this has been challenging but this combustion process results in ultra low emissions and has been achieved over a wide range of engine operating conditions, even from cold start.

The detailed research has so far focused on lower speed and load conditions that represent a major proportion of an engine's operation in a real world environment. At 2000rpm and up to approximately 2.7 bar IMEP (Indicated Mean Effective Pressure), the ISFC (Indicated Specific Fuel Consumption) achieved is approximately 10% better than current spray-guided direct injection, spark ignition engines. Emissions results are an impressive 20 ppm NOx at less than 2.3 bar load and has four-stroke-equivalent hydrocarbons and carbon monoxide emissions.

Simon Wood, Technical Director of Lotus Engineering said: "These impressive results represent an important step-forward in Lotus Engineering's strategy of developing an array of more efficient multi-fuel combustion systems. Omnivore lays the foundations for a novel and pragmatic vision of a variable compression ratio engine concept suitable for production. A multi-cylinder version is practical for a wide variety of vehicles and offers greatest benefit to C and D class passenger cars which can take advantage of the low cost architecture and significantly improved fuel economy and emissions. We are continuing our discussions with other manufacturers and eagerly anticipate the development of multi-cylinder demonstrations of this revolutionary engine configuration."

The Omnivore engine concept achieves wide-range HCCI combustion and low CO2 emissions through the application of a simple wide-range variable compression ratio mechanism, itself facilitated by the adoption of the two-stroke operating cycle. Technologies combined in this package are all synergistic and provide a route to the efficient use of alternative fuels, accelerating the displacement of fossil fuels.

Jamie Turner, Chief Engineer of Powertrain Research at Lotus Engineering said: "The automotive industry, including Lotus Engineering, has quite rightly advocated engine downsizing for four-stroke engines. This is as a result of the dominance of the four-stroke cycle in the automotive world and its generation of throttling losses at part-load, where vehicles run most of the time. The two- stroke cycle, conversely, does not suffer from significant throttling losses and in many ways is a more natural fit for automotive use. With the thermodynamic disadvantages of throttling losses removed, the two-stroke engine is free to be sized according to its improved part-load fuel consumption. Downsizing therefore isn't vital and, due to the improved light-load efficiency and emissions performance we see with Omnivore, this technology approach and 'upsizing' could permit a more efficient engine."

The initial Omnivore programme has been in collaboration with Queen's University Belfast and Orbital Corporation Limited Australia, with sponsorship from DEFRA/DECC and DOE NI through the Renewables Materials LINK programme. Future work by Lotus Engineering will concentrate on further investigating the operation on gasoline and alternative renewable fuels such as ethanol and methanol, with more in-depth analysis of specific test points.

Technical Detail

Omnivore Summary

The Omnivore engine concept features an innovative variable compression ratio system and uses a two-stroke operating cycle with direct fuel injection. It is ideally suited to flex-fuel operation with a higher degree of optimisation than is possible with existing four-stroke engines.

The engine concept features a monoblock construction that blends the cylinder head and block together eliminating the need for a cylinder head gasket, improving durability and reducing weight. In this case, the application of a monoblock is facilitated by the absence of the requirement for poppet valves. A novel charge trapping valve in the exhaust port allows asymmetric timing of exhaust flow and continuous variation of the exhaust opening timing.

The Omnivore engine uses the Orbital FlexDI fuel injection system which produces fine in-cylinder fuel preparation irrespective of fuel type and, together with air pre-mixing, allows efficient two-stroke combustion and low- temperature starting, whilst offering singular opportunity for advanced HCCI control.
The variable compression ratio is achieved by the use of a puck at the top of the combustion chamber. This simple, yet effective system moves up and down effecting the change in geometric compression depending on the load demands on the engine.

Engine Concept Features Monoblock

The monoblock incorporates the cylinder head, the cylinder barrel and the inlet ports, together with mounts for the variable compression ratio system and the charge trapping valve housing. It also contains the non-moving location of one of the two possible injector mounting positions provided for research purposes. The other injector position is in the variable compression ratio puck. The monoblock is mounted on the upper crankcase, which is a common component with all of Lotus' single-cylinder research engines. The engine carries a full primary and secondary balancer system. The monoblock is water- cooled by an electric water pump.

Computational fluid dynamics is used extensively to ensure effective cooling of the monoblock, a feature assisted by the removal of the cylinder head gasket, inherent in such architecture. The chief advantage of a monoblock construction in any engine, aside from the bill of materials and assembly benefits, is the reduction of bore distortion afforded by the removal of cylinder head bolts. This is especially important in piston-ported 2-stroke engines.

Variable Compression Ratio Mechanism

The primary component of the variable compression ratio mechanism is what is termed the 'puck', or a moveable junk piston in the cylinder head. In the case of the research engine, this puck is driven in and out by a double- eccentric mechanism itself comprising proprietary parts. The puck itself does not move at engine speed. In addition to the spark plug, the puck carries one of two possible injector positions. It is water-cooled and carries simple piston (or 'junk') rings for primary sealing, and an 'O'-ring towards the top for final sealing.

The variable compression ratio system is controlled by an electric motor and worm drive arrangement at the front of the engine. Because there are no poppet valves in the engine, it is clear that the puck could be of a large diameter and since there is no need for valve cut-outs in the piston crown, the minimum volume of the combustion chamber can be much smaller than has been the case in variable compression ratio engines shown to date. When the puck is in its innermost position, its surface is essentially coincident with that of the combustion chamber squish band and this yields the highest compression ratio of 40:1.
The combustion chamber geometry necessarily alters as the puck is moved to vary the compression ratio. The chamber geometry in Omnivore was therefore chosen on the basis of 2-stroke experience in spark ignition operation. Consequently, the puck is positioned in the cylinder head in such a way that the non-moving squish band directs cooling flow towards the spark plug. The puck is water-cooled from the main engine cooling circuit.

Charge Trapping Valve

The charge trapping valve is caused to oscillate by a short articulated connecting link from an engine-speed eccentric shaft itself rotated by a belt drive from the crankshaft. A simple charge trapping valve mechanism provides for asymmetric exhaust timing and hence a modification of the original piston- ported two-stroke operating cycle. Fitting an articulated link between the eccentric shaft and the trapping valve actuating arm affords the opportunity independently to vary the opening and/or closing point. In this 'variable' form, at light load, the charge trapping valve can be made to control exhaust port opening, to maximize expansion in the cylinder, and the blowdown period can be optimised. The position of the control arm is controlled by the engine management system. All charge trapping valve components and their configuration have been analysed kinematically, and since they operate with modified simple harmonic motion, they do not suffer from jerk stresses.

Other Components

The cranktrain of the engine comprises an 86 mm stroke crankshaft, a trunk piston of 86 mm bore and a connecting rod with 195.5 mm between centres. The piston carries four piston rings: two pegged half-keystone compression rings which traverse the ports in the upper section, and a Napier scraper ring and U-Flex oil control ring in separate grooves in the lower portion. These are not pegged since they do not have to traverse the ports. In this manner, the working chamber is completely sealed from the crankcase and hence wet- sump lubrication can be employed.

Since this is a research engine, it is cooled by an electric water pump with a separate electrically-driven oil pump used for lubrication. Scavenge air is provided externally. For convenience, air for the Orbital air-assist DI system is provided from the factory air supply regulated to 6.5 bar maximum air delivery pressure. Note that in any multi-cylinder application it is envisaged that all these subsystems would be incorporated into the engine in the normal manner.

Technical Specification

Configuration Operating cycle Combustion system Bore x Stroke Displacement Compression ratio Porting arrangement
Ring pack Fuel system

Single-cylinder loop-scavenged 2-stroke Modified two-port two-stroke cycle Homogeneous charge compression ignition 86 mm x 86 mm
499.6 cc 10:1-40:1 Schnürle loop scavenging with five transfer ports and one exhaust port fitted with Lotus charge trapping valve providing asymmetric and continuously-variable exhaust port timing Two pegged half-keystone compression rings in upper section, unpegged Napier and U-Flex oil control rings in lower section for oil control and containment Orbital FlexDI air-assist
10 December 2009
Page 5 of 6
Scavenging air supply Lubrication system Cooling system
Via external rig; adjustable flow
Wet sump with plain bearings and electric oil pump Electric water pump
Indicated Specific Fuel Consumption comparison between Omnivore and leading Homogeneous GDI and Spray Guided GDI engines
Omnivore % Improvement – Indicated Specific Fuel Consumption
IMEP Homogeneous GDI1 Bar
1.02 41.06 1.15 36.43 1.35 36.62
1.5 35.05 1.9 30.45 2.3 27.83 2.5 25.68 2.7 22.89
References
Spray Guided GDI2
10.49 7.91 8.23 4.35 -1.86
1. Coltman, D., Turner, J.W.G., Curtis, R., Blake, D., Holland, B., Pearson, R.J., Arden., A. and Nuglisch, H. Project Sabre: A Close-Spaced Direct Injection 3-Cylinder Engine with Synergistic Technologies to achieve Low CO2 Output
SAE paper number 2008-01-0138
2. P. Luckert, A. Waltner, E. Rau, G. Vent and U. Schaupp The new V6 gasoline engine with direct injection by Mercedes-Benz. MTZ Vol 67, 11/2006 Pages 830 - 840


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    • 1 Second Ago
  • 14 Comments
      • 5 Years Ago
      Love this kind of cutting-edge engineering.

      Very fascinating stuff. It will be interesting to see who is the first to bring an HCCI to market first - besides Lotus, Mercedes and GM have both been researching the technology for years now.
      • 5 Years Ago
      I wonder when we get to see the first dyno results, and first production prototype. I would expect this to produce nice hp as well.

      How's the durability?
      • 5 Years Ago
      Proof that the internal combustion engine is far from dead.
      • 5 Years Ago
      I was wondering when they will use this tecknowlegy in two stroke engines. I imagine a two cylinder two stroke will perform like a four cylinder four stroke.
      • 5 Years Ago
      So, is this pretty much the closest thing we're going to have to a Mr. Fusion for the foreseeable future?
      • 5 Years Ago
      MAN made a multifuel engine back in the 50's and sold it to the army. The army's M35A2 and some 5-ton had an Inline-6 multifuel diesel that doesn't have any electronic controls, 100% mechanical. It used a device called a fuel density compensator that adjusted the amount of fuel given based on it's viscosity, plus special heads that caused higher compression. It ran off jetfuel, diesel, gasoline, used engine oil, transmission fluid, hydraulic fuel, vegetable oil, heavy fuel oil, and pretty much everything except aviation gasoline, methanol or ethanol, those have too high of an octane rate, it'll run modern E15 if you add about a quart of motor oil to the gas tank and it wont run E85, not enough lubrication to the injection pump. I have a 1971' Kaiser Jeep M35A2 with a Continental Multifuel. This is a 4 stroke diesel type engine with special heads and an FDC to allow it to consume most fuels, it has gear driven timing and no computer controls. The turbocharger boosts the cylinder pressure to help burn fuel more completely, although it'll still smoke from unburned fuel under a heavy load.

      Modern FMTV medium trucks are straight diesel that are computer controlled, they actually manage to be less efficient than the mechanical M35A2 but the engines have a longer lifespan and dont need to be replaced as often as the multi. The multifuel has a max rpm of 2800, and if you exceed that the case iron crankshaft could shatter, if you want to go higher than 56-60 mph on stock tires you have have to exceed the do not exceed speed, soldiers tend to do that in moments of danger, thus the life of the engine is shortened.
      • 5 Years Ago
      2-stroke engines have an extremely high hydrocarbon output. That's why some mopeds are much worse for the environment than SUVs. Unless they've magically found a way around this (I'm not sure how you could neutralize this without expensive equipment), this is not a tenable technology...
        • 5 Years Ago
        I'm not a 2-stroke expert, but I would expect that most of that high hydrocarbon output in a traditional 2-stroke is because of fuel short circuit. Direct injection should all but eliminate fuel short circuit.
        • 5 Years Ago
        Read the press release. There is no magic and nor is it new technology. The engine uses a loop scavenge type of induction, similar to that used on older 2-stroke diesels. Crankshaft lubrication is the same as in a "conventional" engine, i.e. wet sump (you can see the oil filter on the lower right of the first image), so there is no need to add oil to the intake charge, therefore low HC output. The Orbital FlexDI fuel injection system atomises the fuel into extremely fine droplets, further reducing HC output.
      • 5 Years Ago
      I have wondered if direct injection would make two stroke more practical for on road engines. Because you would only have air being taken into the cylinder instead of an air/fuel mixture, you would not have unburned fuel being blown out the exhaust. One of the major disadvantages of two stroke engines.
      • 5 Years Ago
      Awesome that Lotus is doing this but I for one don't think making a two stroke AS good as a four stroke will merit their use in cars. This is a great test bed for this tech since there isn't a big valvetrain to worry about (thus a movable puck for varying compression) but it will be truly useful and amazing when they squeeze this setup (or most aspects of it) into a DOHC engine. Although how they could possibly do that is beyond me which is why I'm a bit cynical.
        • 5 Years Ago
        This can't be done in a 4stroke setting, simply because the puck would need to be moved into the piston as this is undoable on overhead valves.

        2stroke has always been better in everything else EXCEPT emissions. Now that their emissions is down aswell, their usage in cars is well worth it.

        2stroke engines have higher efficiency, can produce more hp per liter, are simpler, way less moving parts etc etc. the benefits just keep on going, but the emissions has been high.

        High emissions caused mostly by not having any kind of valves, and therefore backpressure etc. is being utilized to keep the mixture in the cylinder. There's always some leakage of mixture into the exhaust. Also the need to use oil in the fuel of traditional 2stroke is a very major blow to it's emissions.
      • 5 Years Ago
      waaaaaaaaaajajajajaja su*k it toyota, honda and all other manufactures, this is true engeneering
        • 5 Years Ago
        Yes, this is true engineering. It's true engineering made public. Honda, Toyota, etc. are probably all doing similar work in their engine research departments, they just aren't talking about it publicly. Honda in particular is known for bringing some pretty wacky engine concepts to production. There isn't really anything new here. HCCI, variable compression ratio, and DI 2-strokes are all ideas that have been around for years. Aprilia has been selling a DI 2-stroke scooter for a long time now.

        Of course I'm not saying that this isn't exceedingly cool. Very technical, very useful work they're doing here.
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