An interesting development from Mazda, who claim to be bringing out a compression petrol engine in 2019. The lump will still have plugs but they are only needed at slow speeds, otherwise the fuel will detonate on compression like a diesel. They claim a 30% reduction in fuel consumption.
www.theguardian.com/business/2017/aug/08/mazda-petrol-engine-breakthrough-puts-another-nail-in-the-coffin-of-diesel
Last edited by: Robin O'Reliant on Tue 8 Aug 17 at 21:19
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But presumably a corresponding increase in nitrous oxide emissions?
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With NoX currently being booed like todays pantomime villain and as welcome as a fart in a lift, I dont think any manufacturer would be foolish enough to unleash new technology that increased it.
Last edited by: Zero on Wed 9 Aug 17 at 08:23
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Why would it increase nitrous oxide emissions? Surely they are dependant on the fuel being burnt and not on how it is ignited.
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Haven't read the detail though one advantage of spark ignition is that you can control precisely when the spark occurs.
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>>Haven't read the detail though one advantage of spark ignition is that you can control precisely when the spark occurs.
With modern injection systems you can precisely control the combustion depending on the timing of the fuel being released - even several injections of fuel per cycle can be managed.
Adds a bit of complexity, like the variable compression engines in development.
Maybe something more simple, say a magnet & wire coil device that could convert stored energy into kinetic energy without any of this faffing about would be a better direction to investigate....
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>> Maybe something more simple, say a magnet & wire coil device that could convert stored
>> energy into kinetic energy without any of this faffing about would be a better direction
>> to investigate....
The problem is off course getting your energy into your storage device.
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>>The problem is off course getting your energy into your storage device.
No problem to me in past 3 months and I've got one of the crappier charge rate EVs (11kW max).
Last edited by: Lygonos on Wed 9 Aug 17 at 10:08
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The quote from the Mazda guy in this article says ""better thermal efficiency, improved fuel economy and lower nitrogen oxide emissions"
www.autocar.co.uk/car-news/industry/groundbreaking-mazda-skyactiv-x-petrol-engines-be-%E2%80%98cleaner-electric%E2%80%99
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Seems to me that any benefits come from developments other than sparkless ignition.
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>> With modern injection systems you can precisely control the combustion depending on the timing of
>> the fuel being released - even several injections of fuel per cycle can be managed.
>>
That's been commonplace for years though the ability to control the spark per cylinder based on input from numerous sensors must be beneficial in efficiency terms.
It's not as if the ignition system is a problem that needs solving, electronic ignition is almost totally reliable these days.
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>> It's not as if the ignition system is a problem that needs solving, electronic ignition
>> is almost totally reliable these days.
But if your fuel consumption really is cut by 30% that is an advantage.
Last edited by: VxFan on Wed 9 Aug 17 at 16:44
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>> But if your fuel consumption really is cut by 30% that is an advantage.
Clearly though as I said before I reckon any benefits come from developments other than sparkless ignition.
Last edited by: VxFan on Wed 9 Aug 17 at 16:44
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"... I reckon any benefits come from developments other than sparkless ignition."
The benefits come largely from the fact that a higher compression ratio makes the engine more efficient.
As an owner of a Mazda CX-5 with the 2-litre petrol engine I realise that sparkless ignition is a logical development for Mazda. The 2-litre petrol already has "the world’s first gasoline engine for mass production vehicles to achieve a high compression ratio of 14.0:1", according to Mazda's publicity.
It's not far off diesel territory. Apparently, specially designed exhaust manifolds, piston heads and fuel injection techniques are all part of preventing "knocking".
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More details from Auto Express:
"The next-gen engine, called Skyactiv-G Generation 2, will adopt a Homogenous Charge Combustion Ignition (HCCI) lean burn ignition system.
The current Skyactiv petrol engine features a diesel-like 14:1 compression ratio. But Mazda’s next petrol takes this to 18:1, which means that the piston’s compression alone can cause the fuel mixture to ignite, just like a diesel.
This works at up to 50 per cent throttle loads, allowing the engine to run very lean amounts of fuel, but with almost no NOx and particulate emissions, making it as fuel efficient as a diesel, but cleaner than a petrol.
HCCI does limit power, so the engine switches over to conventional spark ignition, with higher fuel to air ratio, when you press the throttle harder."
tinyurl.com/y8b7tan7
Note comments about very low NOx and particulate emissions.
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>>The current Skyactiv petrol engine features a diesel-like 14:1 compression ratio.>>
Though an 11ish to 1.5 turbo would probably be more flexible and more economical.
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>>HCCI does limit power, so the engine switches over to conventional spark ignition, with higher fuel to air ratio, when you press the throttle harder."
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So it has an ignition system, I still can't see the benefit of not using it under lean burn conditions.
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It's not necessary with high compression: ignition begins with the injection of fuel at these ratios. Also sparks create NOX as they react oxygen and nitrogen present in the air.
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>> It's not necessary with high compression: ignition begins with the injection of fuel at these
>> ratios. Also sparks create NOX as they react oxygen and nitrogen present in the air.
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Yes I know it's not necessary. That's not my point. My point it that I would have thougjt it woulld be beneficial to control the precise point of the ignition when in lean burn as the engine needs an ignition system anyway ...
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All air/fuel mixtures spontaneously detonate at a certain compression, but the crucial feature of old diesels was that only the air was compressed, and then the fuel was injected straight into the combustion chamber at the precisely determined moment.
Petrols worked on a different principle, and the whole fuel/air mixture was compressed, but not as much as with a diesel so it didn't quite explode, until triggered with a spark.
I'd understood that the reason the diesel practice couldn't be applied to petrol was that petrol detonated even more violently than diesel, being much more volatile. So this development must have overcome that problem. But it's still only with a lower compression ratio than a diesel, which is in the 15 -23 : 1 range not the 14:1 quoted here.
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"So this development must have overcome that problem. But it's still only with a lower compression ratio than a diesel, which is in the 15 -23 : 1 range not the 14:1 quoted here."
14:1 is the compression ration used in the current range of Mazda SkyActiv petrol engines. The HCCI next generation (so-called sparkless) engines will be 18:1, which puts them firmly in diesel territory.
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Your point is noted, but ultimately pointless ;-)
The ignition begins and is thus timed by the injection of fuel - sparking a moment early does nothing as there is no fuel, a moment later and combustion has already begun.
It's kinda the point of compression ignition systems
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>> Your point is noted, but ultimately pointless ;-)
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>> The ignition begins and is thus timed by the injection of fuel - sparking a
>> moment early does nothing as there is no fuel, a moment later and combustion has
>> already begun.
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>> It's kinda the point of compression ignition systems
>>
You have a slightly patronising way sometimes Lygonos, as though you think you are an expert and we simply laymen.
Relying on compression ignition in a petrol engine accounting for all the vagaries around fuel quality/characteristics, air quality, humiidity, temperature etc still seems a little odd to me. Better to inject and ignite totally precisely via high pressure DI and a spark. I am wondering if it uses compression ignition when cruising and a spark under load. However I am sure there is more to it than meets the eye as the Guardian article is at best ambiguous.
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"I am wondering if it uses compression ignition when cruising and a spark under load."
Yes.
"The Skyactiv-X will also be supercharged, and despite earlier reports, it retains its spark plugs for igniting the air-fuel mixture under higher loads like during warm up for full throttle running."
(from tinyurl.com/ybn5xdk7)
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My 1.4 petrol uses 2 cylinders under lighter loads to save fuel. Best run I've had so far (once) is 48MPG. That's mostly motorways. Maybe it's my driving style but the best I got in a 170PS diesel Passat (admittedly heavier) was 46MPG.
The long term average for the Passat was 36MPG (due to lots of shorter journeys) and the Audi is near 35MPG. I am so glad I went back to a small turbo petrol engine :-)
But then I've chosen the same for the next car. When it finally gets built and delivered. Looking at early December!
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>> "I am wondering if it uses compression ignition when cruising and a spark under load."
>>
>> Yes.
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>> "The Skyactiv-X will also be supercharged, and despite earlier reports, it retains its spark plugs
>> for igniting the air-fuel mixture under higher loads like during warm up for full throttle
>> running."
>>
>> (from tinyurl.com/ybn5xdk7)
>>
Thanks FP,, that's making more sense, it will be the transition as the driver comes on and off the throttle that will be the challenge, I reckon there will be a short threshhold where it retains the spark before "switching" to CI on a steady throttle or cruise control conditions.
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>> Relying on compression ignition in a petrol engine accounting for all the vagaries around fuel
>> quality/characteristics, air quality, humiidity, temperature etc still seems a little odd to me.
One of those vagaries is that a petrol/air mixture can explode prematurely (pinking) if the compression ratio is too high, the petrol is of too low an octane rating, or the engine is under heavy load such as hard acceleration or pulling up a hill.
This risk limits the compression ratio in a traditional carburettor system, and also with injection if that is only into the inlet manifold.
Direct injection straight into the combustion chamber removes that limitation, and the two fuels become theoretically interchangeable - except of course that diesel fires entirely under compression alone, whereas this development seems to combine both spark and compression ignition.
But as Lygonis so succinctly points out, if both the injection and spark occur at precisely defined moments, it is hard to see how there is any scope or purpose in varying them.
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except of course that diesel fires entirely under compression alone, whereas this
>> development seems to combine both spark and compression ignition.>>
Which suggests, as I suspected, that compression ignition is not satisfactory alone in a petrol engine.
>> But as Lygonis so succinctly points out, if both the injection and spark occur at
>> precisely defined moments, it is hard to see how there is any scope or purpose
>> in varying them.>>
In a petrol engine it is desirable for the combustion to commence before TDC and the timing of the spark, and the injection, is varied to a very fine degree depending on load and numerous other factors. Benefits of higher octane fuels include reduced likelihood of pre-ignition/knock/pinking, so the ignition can be more advanced, and the burn is more controlled and more complete so more power is extracted. Accordingly it doesn't seem ideal to rely on compression ignition the timing of which will be conversely influenced by fuel quality, etc, etc. I.e. On face value higher octane fuel would bring efficiencies via advanced ignition timing though would theoretically ignite later under compression ignition. There is much more to learn.
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I'm by no means an expert in all of this, but my understanding is that control of the combustion chamber temperature has an extremely important role to play in preventing knocking. Most of the unwanted high temperatures in a high compression-ratio petrol engine are the result of exhaust gases lingering after combustion.
Much of Mazda's efforts have been focused on improving gas flow by managing the effect of pressure waves in the exhaust. The very long exhaust manifold, in which gases from two cylinders are combined before being combined with the remaining two cylinders (4-2-1 system) is part of this.
This, apparently, is so effective that the exhaust gases are too cool to activate the cat fully. Mazda's solution to this is a specially-shaped piston head with a pronounced cavity beneath the spark plug (producing a self-contained volume of air/fuel mixture) and retarding the spark. This also contributes to faster ignition.
All the above is taken from Mazda's publicity on their current SkyActiv petrol engine with 14:1 compression ratio. It seems reasonable to make some assumptions from this about how Mazda has moved on to utilise an 18:1 compression ratio.
(tinyurl.com/hjmd6fn)
Last edited by: Focal Point on Thu 17 Aug 17 at 09:43
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Similar piece more specific to the latest MX5 engine:
www.roadster.blog/2015/02/mx-5-skyactiv-g-engines.html
The MX5 has a 13:1 CR, which might or might not be related to a shortened 4-2-1 exhaust manifold.
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This chap knows his biscuits on all things motor related. Here's his take on HCCI engines from Mazda.
www.youtube.com/watch?v=9KhzMGbQXmY
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>> This chap knows his biscuits >>
That makes a lot of sense. I am not sure about the point he makes regarding running a lower compression (CR) under spark ignition, after all spark ignition will be deployed under load which is when power and torque will be the objective and the supercharger will increase the effective CR.
Also the final point about spark assisted compression ignition (CI) really only describes what happens in a contemporary spark ignition system.
I think that the default will be a high CR spark ignition engine perhaps with the spark plug shielded or even in a plenum like the latest F1 engines. Upon cold start and under load the engine will run conventionally though on the cruise the mixture will be leaned off which will increase the temp in the combustion chamber (CC) to the extent that CI will happen, then when the driver wants power the mixture will be made richer and the spark will take over, initially a little retarded though will be advanced as the CC cools - all in a very short period of time though the throttle response may be a bit odd.
Last edited by: Hard Cheese on Fri 18 Aug 17 at 14:02
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To expand on the above - the latest F1 engines achieve power and efficiency by having the spark plug in a small chamber into which fuel is directly injected, the mixture in the chamber is rich enough to be ignited by the spark and then as the flame spreads into the CC the weaker mixture in the CC is ignited by the flame. This also provides a slower, more consistent push on the piston crown rather than a less well controlled bang thus improving power, relevant even at 15000 rpm and more or 125 firing pulses per second per cylinder.
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Sparkless Petrol Engine - Fursty Ferret
