The robust aftertreatment that is required for a 2-stroke has pretty much made this idea non-viable. However, since we are being forced into greater aftertreatment for everything, that may become less of an issue. The article does have a sales-pitch feel, though, raising the reading on my skepticism spidy meter.
Here's a little Flash demo of the engine. Orbital have quite an interesting suite of technologies available, as DI becomes more popular I think their long experience is going to be useful.
There are examples of 2-stroke DI motors with very very clean emissions, cleaner than the 4-strokes they compete against in those particular classes. Particularly outboard boat engines and snowmobiles.
@Junior37c - for some of the currently lightly regulated applications, sure. But not for automotive applications, and an increasing number of industrial and other applications.
With the recent pace of emissions regulations, it's a use-it-or-lose-it world. Once you give up on a particular system (e.g. automotive diesel in passenger cars) it's very difficult to re-enter, although clearly not impossible since that particular one has been accomplished (although it's still uncommon).
The Lotus website is very good - maybe the best I have seen.
The engine presumably needs a blower for intake/scavenge etc. - so this is a bit awkward and complicates things a little. Also presumably - they probably need the variable CR to make the engine comparable to a 4-stroke in fuel efficiency - but again this takes away from the 2-stroke engine's advantage of simplicity. It would probably not be cheaper, lighter or simpler than a 4-stroke. This is interesting but probably not the future of the IC engine.
I can see a parallel in design with the Wankle rotary design I first saw in the early 60's. I tried my first rotary powered car in 66, an NSU from a dealer in Long Beach. Funny little car that, at the time, I considered a waste of time.
From that experience I can see a possible future for this two stroke design...Much the same in 2040 as the rotary engine is today.
I'm not terribly up to date on modern automotive emission specs but maybe we can start to quantify this. The DI snowmobile engines I'm used to dealing with have no catalytic aftertreatment and emission numbers are approximately as follows on a production engine. This is also not a variable compression ratio engine.
Unburned Hydrocarbons
5.5g/kwh
CO
30g/kwh
CO2
0.27g/kwh
NOx I didn't note down from the paper as it was something ridiculous like 95% under what a comparable 4-stroke was running. Off the top of the head I think it was in the range of 2-3g/kwh
Any input on how close this is to automotive regs?
The variable CR could play hell with NOx emissions.
Auto emissions are done according to a total drive cycle, not normalized per kWh. Tier 2 bin 8 (highest emission allowed currently and not good enough for California) allows 0.125 grams per mile HC. Bin 5 (max for Calif) allows 0.090 grams per mile of HC. So, unless you can go 61 miles on only one kWh of engine output power (not very likely!) those are nowhere near good enough. HC and CO are easy to clean up with a catalytic converter, NOx isn't. Two-strokes often have a NOx advantage because the leftover exhaust from the last cycle acts like EGR and dilutes the combustion. But the allowed amount is only 0.2 g/mi for bin 8 and 0.07 for bin 5.
HCCI is supposed to be good for NOx because it allows operation with a dilute charge and a lot of EGR, which reduces peak combustion temperature. The variable compression ratio is the trick they're using to get self-ignition to happen at the right crank timing.
A direct comparison can be made on the heavy truck emission standards, which are normalized to hp-hr (which is about 35% different from a kWh). The current standard is 0.14 g/hp-hr for non-methane HC, which is a long way off from the engine-out emissions of the snowmobile engine.
hmm, I see. I suspect that a catalytic could bring the UHC down to ~1g/kwh but much beyond that is pushing it. Possibly going to a stratified charge as well as after treatment could get us there. But the target is a moving on, by the time that could be sorted the regulations will have shifted again.
What an odd mishmash of units they use, grams/hp hour.
I'm sure they can make an aftertreatment system that gets those emissions into the box, probably combined with even greater tweaks on the initial system to get the base combustion emissions lower. The issue is/was that any such aftertreatment system would be maybe 2-stage with some active components, etc. That would have been absolutely exotic 10 years ago, and probably still not commercially competitive today, but that may not be the case in a few years. If the overall efficiency is better and the technology comes to the point where it's at least a question that it might be viable, someone will surely take a look.
The emissions of the OMNIVORE engine are comparable to 4-stroke in terms HC and CO due to controlled HCCI combustion at part loads. This is great, but they fail to state the emissions at full load with SI. I think they try to improve the numbers somewhat by using higher alcohol content for the full power figures, but fail to offer those emissions numbers for comparison. So I dont see the point of a 'flexible' engine that still cannot operate at full load within emissions targets.
I saw the engine presented at a conference recently, and the discussion about full load emissions was not really concluded.
If not for the emissions laws though, it is quite a little powerhouse.
At full load, you're not running in lean-burn mode, and with direct injection, there's no direct loss of fuel to the exhaust ports. At full load, one would expect that the emissions wouldn't be too different from a 4-stroke engine. All the stuff still has to go through a catalyst to clean it up.
And on that point ... 4-stroke spark-ignition engines usually intentionally run somewhat rich at full load to protect pistons and exhaust valves. The standard automotive EPA drive cycle does not include any full load operation at all, and it's possible to arrange for the more aggressive US06 to not include full load operation, either. Some underpowered vehicles may need intermittent full throttle for that one, but you can game that by operating stoichiometric for a certain time delay then going rich to protect engine components. This isn't really much of an issue since existing 4-stroke engines have a problem with full load operation anyway. If anything, the 2-stroke operation will allow a bit of air out the exhaust ports to allow overall lambda=1 even though in-cylinder lambda can be slightly rich if it's necessary to protect the piston.
One other thing that two-strokes have trouble with, is that the ports in the cylinder wall inherently let some lubricating oil out the exhaust, even on engines without crankcase scavenging. Don't know how they're handling that - this is a trickier problem than full load.
I think even with the best DI system, and the VERY strange shape of combustion chamber and piston bowl they used, they still struggled with full load emissions. I.e. still alot higher than 4 - stroke. CATS dont like fuel on them either....
I think they had pretty much solved oil loss past the rings at the port, saying that it was no greater than a modern 4-stroke. The setup is a bit exotic though and I wonder how they would still seal at 100,000 miles.
"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"
I did not see what is responsible for intake functions, now that the crankcase is a wet sump. Maybe the gray box off to the left that leads to some of the transfer ports is an air compressor?
