IN­SIGHT

Poorly ad­vised politi­cians want to force us into elec­tric cars and hy­brids, but Mazda’s rev­o­lu­tion­ary new tech­nol­ogy proves that petrol en­gines still have a bright fu­ture

Total MX-5 - - CONTENTS -

There’s life in the petrol en­gine yet. We take a look at Mazda’s ground­break­ing new Skyactiv-x en­gine

Apetrol en­gine has its fuel ig­nited by a spark, while a diesel en­gine’s fuel ig­nites when the air into which it is in­jected is com­pressed so hard that it gets hot enough to set the fuel on fire. But what if you could set petrol alight the same way? And why would you? Would it make the en­gine bet­ter – more pow­er­ful, more ef­fi­cient, cleaner? And if it did, why has no-one done it be­fore?

You’re read­ing about this in a mag­a­zine fo­cused on a par­tic­u­lar type of Mazda be­cause the man­u­fac­turer of our favourite Ja­panese sports cars has al­ready blurred the petrol/diesel bound­ary more than any other car­maker. A mod­ern petrol en­gine usu­ally runs with a com­pres­sion ra­tio around 10:1 with in­di­rect fuel in­jec­tion, or maybe as high as 12:1 with di­rect in­jec­tion in which the squirt of fuel cools the air be­ing com­pressed in the cylin­der enough to avoid the pre-ig­ni­tion (knock­ing) that would oth­er­wise oc­cur. Mod­ern tur­bod­iesels typ­i­cally run at around 17:1.

Mazda’s Skyactiv mo­tors as launched in 2011, though, run at 14:1, be they petrol or diesel. For the Skyactiv-g petrol en­gine, this means a big­ger bang each time the fuel ig­nites, so more power for less fuel. For the Skyactiv-d diesel, it means the fuel can be in­jected ear­lier and can burn for longer, so re­leas­ing more power with­out get­ting too hot too soon and ig­nit­ing pre­ma­turely.

Mazda’s new Skyactiv-x petrol en­gine, planned for pro­duc­tion dur­ing 2019 and ini­tially to be fit­ted to the Mazda 3, takes the blur­ring a stage fur­ther. One of the rea­sons a diesel en­gine is very fu­el­ef­fi­cient is that it can run with a very lean fuel/air ra­tio. The heat gen­er­ated by com­pres­sion is enough to ig­nite the fuel, even if there is an ex­cess of air which doesn’t get used in the burn­ing process. In a petrol en­gine, though, the air and petrol need to be mixed in a ra­tio some­where near the chem­i­cally-cor­rect one (14.7:1), the one that en­sures all the petrol and all the oxy­gen are used up, if the mix­ture is to be suc­cess­fully ig­nited by a spark.

But if the petrol can be in­jected into air in the cylin­der that’s be­ing com­pressed even more than in the Skyactiv-g, by means of both a mas­sive 15:1 com­pres­sion ra­tio and a su­per­charger, then it’s close to be­ing ig­nitable with­out a spark. And in­stead of a flame prop­a­gat­ing from a sin­gle point (the spark plug), there will be si­mul­ta­ne­ous com­bus­tion right through the petrol/air mix even if it is much leaner – maybe as lean as 30:1 when cruis­ing, to the great ben­e­fit of econ­omy.

This sounds great in prin­ci­ple, but petrol burns more quickly than diesel and con­trol­ling the com­bus­tion in a com­pres­sion-ig­ni­tion en­vi­ron­ment is very tricky. The petrol might not ig­nite, or it might ig­nite too soon if there’s too much fuel, caus­ing the ‘pink­ing’ of preig­ni­tion. Please meet, then, the com­pres­sion-ig­ni­tion en­gine with spark plugs: Skyactiv-x.

Here lies the Skyactiv-x break­through. It uses the ex­pand­ing fire­ball around the spark plug, as it ig­nites the petrol/air mix, as an ‘air pis­ton’ to in­crease pres­sure in the rest of the com­bus­tion cham­ber and so trig­ger com­pres­sion ig­ni­tion in the rest of the mix, how­ever lean. This sounds like a recipe for in­stant pink­ing, and in a sense it is a very

care­fully ma­nip­u­lated ver­sion of ex­actly that. The se­cret lies in con­trol­ling – by means of com­puter power un­think­able a few years ago – ex­actly when it hap­pens by al­ter­ing the tim­ing of both the spark and the in­com­ing squirt of fuel into the fast-com­press­ing air within the com­bus­tion cham­ber.

But as well as this squirt, Mazda’s

SPCCI sys­tem (Spark-con­trolled Com­pres­sion Ig­ni­tion) also gives a squirt dur­ing the in­duc­tion stroke. The mix­ture this cre­ates is too lean to ig­nite ei­ther by com­pres­sion or spark on its own, but it means that at least some petrol is al­ready dis­trib­uted through the new slurp of air as it’s com­pressed. And that means the in­jec­tors don’t have to squirt quite as much petrol in their sec­ond pulse, on the com­pres­sion stroke, in the tiny time they have to do it. Pres­sure­wave sen­sors de­tect if pre-ig­ni­tion is start­ing to oc­cur, and the tim­ings are al­tered to stop it.

This com­pres­sion ig­ni­tion mode cov­ers most of the en­gine’s run­ning, but the lean mix­ture doesn’t burn quickly enough for power at high en­gine speeds. That’s when the ig­ni­tion and in­jec­tion tim­ing, and the amounts of fuel in each squirt, are al­tered to get the mix­ture com­bust­ing sooner, so the en­ergy of its ex­pan­sion doesn’t dis­ap­pear down the ex­haust pipe. It’s now run­ning as a nor­mal spark-ig­ni­tion en­gine which means that, un­like a diesel, the

Skyactiv-x can rev in the way we like petrol en­gines to rev.

Mazda prom­ises a par­tic­u­larly keen throt­tle re­sponse as well as a ‘dras­tic im­prove­ment’ in fuel econ­omy. Early re­ports sug­gest that the 2.0-litre ex­per­i­men­tal en­gines so far sam­pled, in mules based on the cur­rent Mazda 3, feel and sound quite ‘nor­mal’ and have im­pres­sive low-end torque. Cur­rently they ‘pink’ too of­ten, un­der sud­den loads like a nor­mal en­gine with over-ad­vanced ig­ni­tion but also, cu­ri­ously, when com­ing off the throt­tle. All will no doubt be fixed; th­ese are early days.

What we want to know, of course, is when the Skyactiv-x en­gine will ar­rive in an MX-5. Mazda rules it out for the cur­rent ND gen­er­a­tion, but it would surely be a part of the NE of­fer­ing should the MX-5 con­tinue in a fifth in­car­na­tion. Mazda won’t say if there will in­deed be a next MX-5, but the com­pany’s ‘Sus­tain­able Zoom-zoom 2030’ man­i­festo makes it clear that ‘driv­ing plea­sure, the fun­da­men­tal ap­peal of the au­to­mo­bile’ will in­volve the in­ter­nal com­bu­tion en­gine for a long time yet. And it’s hard to imag­ine this Mazda fu­ture with­out some sort of MX-5, es­pe­cially given the sports car’s promi­nence in the man­i­festo’s pho­to­graphs of peo­ple hav­ing a good time with cars.

The com­pany’s tar­get is to re­duce its cor­po­rate av­er­age well-to-wheel CO2 emis­sions to half of 2010 lev­els by 2030, and by 90 per cent by 2050. Elec­tri­fi­ca­tion will play a part, but Mazda’s view is that the big­gest gains will come from im­prov­ing the ef­fi­ciency of in­ter­nal com­bus­tion en­gines be­cause they will ‘help power the ma­jor­ity of cars world­wide for many years to come’.

Above: Skyactiv-x is the fu­ture of Mazda’s petrol en­gine range com­pres­sion ra­tio of 15:1, made pos­si­ble by the new en­gine’s abil­ity to ig­nite a highly com­pressed fuel and air mix­ture. Power, econ­omy and emis­sions all ben­e­fit

Be­low & right: Looks fa­mil­iar, but this is the first sig­nif­i­cant de­vel­op­ment in petrol en­gine tech­nol­ogy for many years. The Skyactiv-x en­gine has a

Skyactiv-x en­gine is cur­rently un­der­go­ing test­ing in a fleet of Mazda 3 test mules. While the new en­gine won’t see ser­vice in the cur­rent MX-5, it will surely fea­ture in the next gen­er­a­tion

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