Mazda to make compression ignition petrol engine.....

Haych

Just seen this on BBC news:

http://www.bbc.co.uk/news/business-40873093

Mazda is set to sell cars with compression ignition petrol engines from 2019, claiming they are 'up to' 30% more efficient than conventional petrol engines.

But, it begs the question why?  Why is petrol compression ignition more efficient than spark ignition?   I'm no engineer so ask the question for education, not to be cynical.

Axe_meister

Lotus have had a compression ignition test bed engine for years.
Does not have a normal camshaft but each valve is moved up and down via it's own actuator. Sound more like a diesel than a diesel apparently

Haych

Hmm, that sounds expensive to repair when it goes wrong. 

Fretwired

Haych said:

Just seen this on BBC news:

http://www.bbc.co.uk/news/business-40873093

Mazda is set to sell cars with compression ignition petrol engines from 2019, claiming they are 'up to' 30% more efficient than conventional petrol engines.

But, it begs the question why?  Why is petrol compression ignition more efficient than spark ignition?   I'm no engineer so ask the question for education, not to be cynical.

I think the compression allows for a more powerful spark meaning less petrol is needed for combustion.

aord43

Fretwired said:

Haych said:

Just seen this on BBC news:

http://www.bbc.co.uk/news/business-40873093

Mazda is set to sell cars with compression ignition petrol engines from 2019, claiming they are 'up to' 30% more efficient than conventional petrol engines.

But, it begs the question why?  Why is petrol compression ignition more efficient than spark ignition?   I'm no engineer so ask the question for education, not to be cynical.

I think the compression allows for a more powerful spark meaning less petrol is needed for combustion.

No spark is needed with compression ignition.  Normal engines compress then spark.  This one apparently just compresses and it ignites due to the compression (because of the heat, I assume).  That's how diesel works, as I understand it.
My guess, and I'm no expert, would be that because under compression it heats up uniformly, it ignites more efficiently than using a spark which is clearly at a single point in the cylinder and the ignition has to spread across the cylinder.
Also possibly the shape of the internal part of the cylinder, with no intrusion from a spark plug, would help.

Haych

That's the bit my mind doesn't understand. A bang is a bang whether by compression or spark. Does some of the petrol vapour in the cylinder say "nope, not going bang, that spark wasn't big enough"? It's a chain reaction surely?
Again, not attempting cynicism, just after clarity. 

aord43

I also don't actually know...maybe later I will look it up.  But I was trying to suggest that ignition due to compression happens "everywhere" across the cylinder - because the air/fuel mix is all being compressed by the same amount everywhere.
Whereas a spark ignites at a single point which has to propagate, so may be less efficient.
Even if I am right, it's probably a gross simplification!

Haych

Hmm, I see your logic. 

Im normally very mechanically minded and I have a very good understanding of IC engines but the fuel efficiency gains from new tech is something that baffles me quite a bit. 

FX_Munkee

It's thermal efficiency. Compression ignition engines have a higher compression ratio, which gives more kinetic energy from the combustion for the same amount of heat.

I think.

Haych

Ok, I can see that. 

How do they overcome detonation? I always thought that was the biggest issue in high compression petrol engines? The fuel tries to detonate at the wrong point in the cycle and bits of engine get spat out in pieces. 

FX_Munkee

Detonation is compression ignition, so they're not going to overcome it
Detonation is bad in a spark ignition system as it creates forces that the crank is not designed to take. Diesels have different crank shapes for this reason.

Danny1969

I had a compression ignition engine on a Kawaski KH125 back in the eighties. Basically I had been caning the arse of it and all of a sudden it started increasing in revs until it was redlining. I turned off the ignition and that didn't do anything ...... so I pulled the HT lead of the top of the engine and that didn't stop it either ..... it no longer needed a spark. In the end I turned off the fuel and pulled away and rode off what was in the carb until it spluttered to a standstill. 

A KH125 is a disc valve 2 stroke, that kind of engine is probably more prone to do that than a regular 2 stroke 

sinbaadi

It will be clever.  It has a conventional spark ignition as well, which does make me wonder if it's all a bit of snake oil to be honest, but I imagine a high compression ratio that will only ignite purely by compression under very specific conditions.  I wonder if they'll have a turbocharger on it too, maybe they will because it's another good way of controlling pressure in the cylinder.

Haych

FX_Munkee said:

Detonation is compression ignition, so they're not going to overcome it

I get that but they will want the fuel to detonate at a very specific crank angle to TDC. As I understand it in conventional spark ignition engines that suffer from detonation due to high compression it happens at the wrong crank angle (some call it pre-detonation or pre-ignition) which thus causes the damage. 

High performance engines that have this issue usually use high octane fuel which is less prone or they start bolting water injection systems on to keep temps down. 

FX_Munkee

Detonation and pre-ignition are different. Detonation is after the spark, caused by compression ignition of left over fuel/air mix. The pinging is caused by the short sharp pressure spike in the cylinder caused by the flame front of the spark ignition meeting the one from the detonation.
Pre-ignition is what it sounds like, ignition before the spark caused by heat and pressure or to little advance on the spark.
Pre ignition destroys cranks. Detonation destroys pistons and rings.
High octane fuel does help by making the mix less prone to ignition from heat.

So I was wrong about them creating detonation

Haych

See, everyday is a school day. Thanks @FX_Munkee ;

Nitefly

Just been reading the article referenced in the F1 thread (https://www.autocar.co.uk/car-news/industry/groundbreaking-mazda-skyactiv-x-petrol-engines-be-‘cleaner-electric’).

It seems to be saying that it will use normal spark ignition when under "high load", and compression ignition when under "lower load".  I'm assuming "high load" is acceleration, and "lower load" is cruising or on the over-run.  They seem to think this will improve fuel economy because they will use a much leaner mixture under the lower load conditions.

Or I might be completely misunderstanding it, I'm no engineer...

chillidoggy

High load will be on acceleration, but also, as speed increases, there is more load on the engine to shove the car through the air. So the engine load varies with speed. I'm interested to learn how it works, sounds a bit clever.

57Deluxe

they should just stick to making light bulbs...

Haych

57Deluxe said:

they should just stick to making light bulbs...

Didn't Mazda make tubes too? Maybe not the same Mazda. 

digitalscream

Why are Mazda the only company out there seriously trying to make step improvements in combustion engine design? Everybody else makes incremental improvements, whereas they do things like this and the rotary engine.

Or is it just that they don't care much about money?

(obviously, I'm not counting all the companies doing EVs)