The Japanese company's new range of Skyactiv-X petrol engines work in the same way as diesel motors. Here's why
15 April 2019

When Mazda developed its Skyactiv-X Spark Controlled Compression Ignition (SPCCI) engine, it perfected one of the most elusive concepts since Nikolaus Otto’s first thumper clanked into life nearly 150 years ago. Why? Getting petrol to ignite in the same way as a diesel by compression instead of a spark had been an engineering holy grail for a long time. 

Auto Ignition, Combined Combustion System and Gasoline Compression Ignition are all names used for similar past projects by others attempting to perfect the same concept. The proper name for the underlying concept behind all of these (including SPCCI) is Homogeneous Charge Compression Ignition (HCCI). 

With HCCI engines, the fuel is evenly – homogeneously – mixed in the combustion chamber. It’s lean, too, with extra air supplied by a supercharger-like pump. Compression ignition exploits the fact that if you squeeze air enough, it gets hotter (try closing the end of a bicycle pump with your thumb). It’s how sparkless diesels work and why they need a much higher compression ratio than petrol – and it’s why some billed the HCCI concept as the merging of petrol and diesel engine technology. 

But why bother and what are the advantages? Plenty, according to many industry gurus. When fired by HCCI, the fuel air charge ignites as a whole throughout the combustion chamber. In a conventional spark-ignition petrol engine, fuel is ignited at the spark plug and then spreads along a flame front. The benefits of doing it the HCCI way include reduced fuel consumption and CO2 emissions and fewer oxides of nitrogen (NOx). 

Back in 2001, Lotus Engineering developed a system where the fuel was ignited by hot exhaust gases re-ingested into the engine, to achieve much the same goal as Mazda. Ricardo also produced research engines, and in 2007 Volkswagen demonstrated its prototype Gasoline Compression Ignition engine based on a FSI petrol engine. Like the Lotus concept, it used high levels of exhaust gas recirculation (EGR) to ignite the fuel. 

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HCCI using hot exhaust gases could only work when the engine wasn’t under much load, because of the smaller amounts of fuel used. For starting and full power, it needed a conventional spark, and a smooth switch between the two proved difficult to achieve. 

SPCCI might sound like a cheat – after all, it still involves a spark. But the difference is the spark is used to ignite a small, fuel-rich ‘detonator’ charge injected directly around the spark plug. The rest of the chamber contains a lean charge brought to the brink of igniting by high compression. When the detonator charge is ignited by the spark, the expanding fireball from it increases enough extra pressure on the main charge to make it spontaneously combust. The pay-off is that SPCCI works when the engine is at full gallop and not just a gentle canter – so during most normal driving. The difference between this and the earlier concepts is that combustion is guaranteed and stable. You might say it’s a spark of genius.

What is lean burn?

Lean burn simply means lots of air and less fuel. In a petrol engine, the perfect mixture to achieve complete combustion of the fuel is 14.6 parts of air to one of fuel (14.6:1). The Skyactiv-X engine has a Rootes-type supercharger on the front of the engine, not to boost power but to provide enough air to burn much leaner at over 29.4:1.

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Comments
12

15 April 2019
Is it on sale? Will this tech allow Mazda to avoid paying Toyota to slip under the EU emission rules?

15 April 2019

HCCI or not, petrol is a fair more to refine than diesel. This is going backwards if cost is involved: where I live at present, diesel is 3.00 US dollars per gallon; petrol is 3.99 US dollars, with summer blends coming, petrol will rise even more. 

So a cleaner burning petrol motor isn't going to lower a driver's fuel bill if the refinery hasn't found a better and more efficient cracking method for said petrol... 

15 April 2019
Real_sluggo wrote:

HCCI or not, petrol is a fair more to refine than diesel. This is going backwards if cost is involved: where I live at present, diesel is 3.00 US dollars per gallon; petrol is 3.99 US dollars, with summer blends coming, petrol will rise even more. 

So a cleaner burning petrol motor isn't going to lower a driver's fuel bill if the refinery hasn't found a better and more efficient cracking method for said petrol... 

 

Dammit!

If only you had provided your product planning advice BEFORE Mazda went ahead and engineered an innovative new petrol engine!

15 April 2019
Real_sluggo wrote:

HCCI or not, petrol is a fair more to refine than diesel. This is going backwards if cost is involved: where I live at present, diesel is 3.00 US dollars per gallon; petrol is 3.99 US dollars, with summer blends coming, petrol will rise even more. 

So a cleaner burning petrol motor isn't going to lower a driver's fuel bill if the refinery hasn't found a better and more efficient cracking method for said petrol... 

Well in these parts petrol is cheaper per litre than diesel so filling a ptetrol car up is cheaper than filling a diesel car up, if a more efficient petrol car could go as far or further than the diesel then it would be cheaper than a diesel to run. This surely is a good thing, especially if its emissions are lower.

15 April 2019

Need to mention Honda as they've already done it with a 2 stoke, would give the details but I've done that in a previous post.

The thing is after all that effort of improving fuel economy they've eneded up putting a supercharger on it which lowers fuel economy.  Then there's the cost, 2.5 litre engine with a supercharger and all that software will maybe take 4 years to pay for itself.

For it to be financially viable for the masses (which afterall was the point of it) it needs to be 1.5 in Mazda 3 for £20,000 not a probable £30k.

typos1 - Just can’t respect opinion

15 April 2019

I've been extremly exited about this engine from the first time I heard about it.The fact it will be combined with Mazda's mild hybrid set up has me excited at the thought of being able to buy a petrol engined Mazda that offers economy not quiet at their diesel levels, but much better than the current petrol range.  I hope the driveability and performance are similarly somwhere in between their diesels and petrols (hopefully closer to their excellent diesels).

xxxx - the launch engine will be a 2.0, with other capacities to follow later and the supercharger is there merely to provide additional air to the combustion chambers to benefit economy, not increase power which is where the efficiency gains would normally be lost.

"Why is http://www.nanoflowcell.com not getting more media attention? It could be the future... Now!"

15 April 2019

OK 2.0 not 2.5.  Superchargers :- force fed engines generally produce more power and also up the economy (comparatively) as you don't have to rev the engine.

If you need a 2.0, supercharger, electric motor and a battery to improve economy by 20% whilst keeping decent performance then you'll end up having to charge big bucks. This is  probably why you get 4WD with the first versions otherwise a £30k Mazda 3 with an economic engine just looks to expensive.

If it only improves economy by 20% a VAG COD will match it whilst being alot cheaper to produce

 

typos1 - Just can’t respect opinion

15 April 2019

Can't say I see the benefits outweighing the negatives from what has been written. Clever engineering agreed, Mazda once again showing engineering prowess and a willingness to go 'left field', but not sure the target Mazda buyer gives a toss. Most consumers want cheap, reliable vehicles first and foremost as their daily driver.

15 April 2019

I'd avoid this engine if I was buying used and likely to go any where near the 100K mileage mark. Using Petrol in a High Compression engine instead of Diesel remove Oil, which is a lubricrant, so I wonder how will the engine will stand up.

I applaud Mazda's efforts and wish them well with this engine, but at a time when cars are going electric I wonder what the lifespan of such an engine design might be? Mazda should pick up the phone to the likes of Land Rover and see if they would buy some of these engines. For the likes of such companies this engine solves and immediate issue with the decline of diesel so it may be a way to help Mazda wipe it's nose on it's investment.

jer

15 April 2019

Differences in the real world. One thing that I always notice with cut out pictures of car engines is just how tall they are. I know they are wet sump etc but I would think there is a huge scope to make electric car like packaging and frontal area by following motoycycle architectures that are so much more compact and lighter.

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