We’re used to engines which pretty much explode the gases inside the chamber: however, there is a way to avoid it, writes Stevie Knight.
Supposing there was a combustion process that didn’t conform to the ‘suck-squeeze-bang-blow’ description? Didn’t, in fact, ‘bang’ in the usual way at all? There’s potentially a lot to gain in efficiency, little-to-no NOx creation, more complete burn of hydrocarbons and less mechanical stress… plus fuel flexibility. All of which are very useful characteristics for the marine industry.
It’s called ‘entry ignition’ (EI) and basically, it takes the two stages – compression and combustion – and splits them across different chambers.
But, surely, efficiency is affected, or cost, or both? “No,” says developer (and ex-NASA scientist) Dr Peter Cheeseman, explaining that instead, each stage “plays to its strengths”.
While it’s possible to achieve this process with just a pair of cylinders, for optimum efficiency, Cheeseman’s concept has four stages – despite this engine being, technically, a two stroke. It begins by introducing air into the first cylinder, where it is partially compressed: then it’s pushed into a second chamber, where it’s squeezed again. Here, the pressure (which could exceed a huge 70 bar) opens a check valve leading into a reservoir. “So far, it’s basically just a two-stage air compressor,” he explains.
At this point “the magic happens”, says Cheeseman. Fuel is sprayed into an adjacent chamber, with enough time for a thorough mixing with the doubly-compressed air before it moves through a slider valve. This opens into the combustion chamber below, igniting as it hits extremely hot gases – in fact, the exhaust valve closes a little early on the previous cycle, so although the pressure is low the cylinder retains enough heat to kickstart combustion.
Normally, add ignition to a premixed charge and the result is a horrible pressure spike… but EI avoids all of this. “When people first encounter the idea there are typical misconceptions… they are so used to pressure pulses as the fuel ignites that the idea of a continuous smooth combustion, with no accumulation, is surprising”, he explains.
The principle gets fairly close to a Brayton cycle: that is, a constant pressure burn, “but in a practical, achievable engine” he underlines. However, understanding how it works requires a good look at both timing and the specially designed slider valve.
Rather than up and down, these valve blades move laterally across the entry port: tiny triangular notches create pinpoint openings that slowly widen as the piston descends, changing the valve’s geometry – timing also controls power output and torque.
As it opens little ‘flamelets’ appear at each of the slots where the gases flow into the main combustion chamber and ignite. “The resulting pattern is something like a multi-headed bunsen,” says Cheeseman.
Significantly, there’s no flame propagating across the chamber, no accumulation of unburned fuel-air mixture, and as it all ignites as it enters, there’s also little pressure fluctuation.
It’s also worth noting that it allows for a continuous path from the pre-mixing area to combustion chamber during ignition, so as the descending piston draws in the flow, the reservoir behind it is also being refreshed. In fact, the slider valve only finally closes to isolate the combustion chamber from the reservoir at the end of the burn. While the hot gases to continue to push the piston downward the injector again sprays in fuel into the mixing chamber.
The main advantage of splitting the cycle is that various elements “can be optimised for those pressure and temperatures of each specific stage”, explains Cheeseman. For example, as the first chamber is an air compressor, it doesn’t have to be thick walled, its temperature resistance doesn’t have to be high, and the surface area can be larger.
The second-stage piston has a rather smaller diameter: it’s less of a pancake in order to lower the heat loss.
However, the third, main combustion chamber is rather different: while in a typical engine fresh air cools the cylinder walls, an entry ignition engine doesn’t have cold air coming in on the intake stroke. “The third cylinder is always exposed to the hot gases,” points out Cheeseman, “there’s no thermal cycling”.
But, he adds: “While for typical combustion turbulence is your friend – and most standard engines they go to lengths to create it to mix the fuel or propagate the flame, this is behind the high heat transfer rates.”
By contrast, the EI flame pattern has little turbulence, and what there is remains localised around the slider valve entry. Therefore this third chamber, while running hot, avoids the extremely labile thermal cycles and pressure spikes common to SI and CI engines.
Given that the gases retain most of the heat, it’s worthwhile pushing them to a fourth, larger cylinder which again expands the burned gases near to atmospheric pressure, wringing most of the energy out of them. The result is two alternating power strokes for each cycle.
There is one area that needs extra support: start up. There are no left-over hot gases to ignite the charge, so a spark plug is introduced into the mixing chamber – though it won’t be needed for more than a cycle says Cheeseman.
Certainly, there are pollution advantages: entry ignition burns a homogeneous charge like an SI engine, avoiding the hot spots and resulting pollution of a diesel cycle. It also burns lean with a minimum air-fuel ratio of 2.8 (lambda) for complete combustion of conventional fuels. This is the sweet spot: above 2.8 and it will have CO in the exhaust, while below 1.7 will see significant NOx “but in between, no clean up is necessary”, he underscores.
It’s also efficient, as entry ignition avoids the knock-limits “so we can choose to go to the highest compression without auto igniting the fuel” he adds.
Further, EI engines have a fairly flat efficiency curve: this seems to go hand-in-hand with a lower compression effort, and so the engines should prove efficient both up and down the speed range. “It promises to be quite a bit more efficient than standard SI or CI at high and low RPM,” predicts Cheeseman.
Importantly, separating the stages means that the engine could burn “pretty much any fuel that mixes with air ”, says Cheeseman, including ammonia, propane and various alcohols – but the extra layer of control also means it could be used for trickier fuels, such as hydrogen.
However, his favourite option is LNG “as it has a very high octane rating, so you can go to high compression ratios and still not risk autoignition”. Further, port to port, it “works out to be a cost-effective solution”. But in his view, longer term, “I think we are looking at synthetic gas”.
While this is currently just “a paper engine” admits Cheeseman, there appears to be rising interest: alternatives are developing but the power density of a combustion engine is still proving hard to beat.
However, as ICE technology “will probably be with us for the foreseeable future”, he concludes that hybrid solutions, matched with cleaner, greener, more efficient and flexible engines – such as entry ignition – are now more necessary than ever.