Lotus unveiled its latest Omnivore prototype engine at Geneva, a turbocharged, variable-compression, wet-sump, two-stroke unit designed to improve engine efficiency while operating on a range of renewable or gasoline fuels. Lotus is about to commission the testing program. The Omnivore draws on experience from an earlier Lotus project.
The monoblock eliminates a separate cylinder head. “The Omnivore is a piston-ported two-stroke with a modified exhaust valve,” Jamie Turner, Chief Engineer of Powertrain Research at Lotus told AEI. The engine uses the Orbital FlexDI direct-injection system, using compressed air to inject the fuel. “That is entirely validated and in mass production in outboard engines, so it’s every bit as durable and reliable as you would need in an automotive engine,” continued Turner.
The monoblock comprises the transfer ports arranged at the bottom, the exhaust charge trapping valve, and the “puck,” which is the key to the variable compression ratio (VCR). Without the valvetrain at the top of the engine, the mechanism can move the puck up and down at the top of the cylinder, thereby varying the compression ratio. The puck can hold both injector and spark plug, although the test variant on display had the injector positioned to one side.
“The puck can be driven in and out very simply via the simple mechanism on the top,” said Turner. “The mechanism has an eccentric arrangement that just moves the puck up and down under a stepper motor control.”
The system designed so far is made from proprietary components, explained Turner. “We wanted to have the authority to move the puck against full cylinder pressure. That probably won’t be needed in a production version, so everything would be very slimmed down, and in actual fact, you could get all the VCR mechanism onto something the same height as anything else on the engine. So we would expect the engine to be slightly lower than a four-stroke.”
The VCR two-stroke allows the gas exchange to be moved away from the head, said Turner. “The puck doesn’t move cyclically, but it can be moved across a very big range. Theoretically, the engine can vary the compression ratio between 8.0:1 to nearly 50.0:1.
"The two-stroke is the solution people reach for when they need high fuel economy, because the friction is a lot lower,” continued Turner. “With a spark-ignition engine, fundamentally you have to control the air mass entering the cylinder as well as the fuel mass; the two-stroke doesn’t suffer from throttling loss. What you need to have with a two-stroke is a means of controlling its emissions well enough because it cannot run stoichiometrically; it cannot run a three-way catalyst because you have excess air. You can have a catalyst to control CO and HC, but you can’t have a catalyst to control NOx as easily, so you want to generate as little as possible even if you have NOx aftertreatment. So you run it essentially in HCCI (homogeneous-charge compression ignition) mode."
The charge trapping valve is a mechanism that continuously varies the opening point of the valve, explained Turner. "We can control the closing point via this mechanism, and that allows us to trap more or less exhaust gas in the cylinder, so we trap the retained heat and residuals in the cylinder with a controllable mechanism. That will influence the compression ratio, but the point here is that we have a completely independent, very wide range compression ratio adjustment. Theoretically, we should get low NOx."
The project is really about improving the combustion and getting better thermal efficiency, overlaid with renewable alcohol fuels, added Turner. "You can get better efficiency on the alcohol fuels now because you’ve got control of the one thing you’d like to change anyway. So, theoretically, you can increase the proportion of vehicle miles traveled on an alternative fuel, which of course helps to displace fossil carbon.”
Lotus is running the Omnivore program in collaboration with Queen’s University Belfast and Orbital Corp. of Australia. Sponsorship has been made available through the UK Government Renewable Materials LINK program.