Tech Torque

4 x 4 Australia - - Contents - FRASER STRONACH

IN­FINITI to Nissan is what Lexus is to Toy­ota, namely an up­mar­ket sis­ter brand. In­finiti makes a dual-range 4x4, the QX80, which is an up-spec and restyled vari­ant of the Y62 Pa­trol.

But this isn’t about the QX80, rather In­finiti’s re­cent an­nounce­ment that it has a vari­able com­pres­sion ra­tio (VCR) en­gine that’s been 20 years in the mak­ing and is due to ap­pear in new In­finiti models as soon as 2018. If this all comes to fruition, it will be an amaz­ing break­through.

VCR en­gines have been one of the holy grails of au­to­mo­tive de­sign, with patents dat­ing back to the 1920s and many work­ing pro­to­types in lab­o­ra­to­ries over the years. How­ever, there’s been noth­ing sub­stan­tial beyond that.

Some of the finer de­tails of In­finiti’s 2.0-litre, four-cylin­der (petrol) VCR en­gine haven’t been made pub­lic, but the in-prin­ci­ple de­sign is clear to see. As with other VCR en­gines, In­finiti’s VCR de­sign ad­justs the com­pres­sion ra­tio so it can make bet­ter use of forced as­pi­ra­tion, in this case a tur­bocharger. It can even ad­just it­self to run more ef­fi­ciently than con­ven­tional en­gines with­out any help what­so­ever from the tur­bocharger.

Tur­bocharg­ers, like su­per­charg­ers, help an en­gine pro­duce more power by feed­ing in com­pressed air; the the­ory be­ing the more air you can pump into and through an en­gine, the more fuel it can burn and the more power it can pro­duce. If you stuff too much air into an en­gine’s cylin­der and then fur­ther com­press it via the pis­ton on its com­pres­sion stroke, the air can get that hot it will ig­nite the fuel. This is def­i­nitely not what you want in a petrol en­gine.

It’s the spark­plug’s job to ig­nite the fuel at a pre­cise point in time and you don’t want the air/fuel mix­ture to ran­domly self-ig­nite. If it does it’s called preig­ni­tion or det­o­na­tion, and it’s po­ten­tially cat­a­strophic for the en­gine. The pos­si­bil­ity of det­o­na­tion can be less­ened by us­ing a tur­bocharger that pumps less air into the en­gine, but that de­feats, at least in some part, the pur­pose of hav­ing a turbo in the first place.

Al­ter­nately, you can re­duce the amount the air is com­pressed once it’s in the en­gine by re­duc­ing the com­pres­sion ra­tio. The trou­ble with this is a lower com­pres­sion ra­tio is detri­men­tal when the en­gine is work­ing with­out the help of its turbo, as lower com­pres­sion ra­tios mean re­duced torque and power. Fur­ther­more, a low com­pres­sion en­gine run­ning high turbo boost pres­sures gen­er­ally equates to a non-lin­ear power de­liv­ery as the en­gine goes from its ‘soft’ off-boost mode to a ‘hard’ on-boost mode – not what you want for drive­abil­ity.

Now you can see where this is head­ing. You de­sign an en­gine that can vary its com­pres­sion ra­tio to suit what the turbo is do­ing or not do­ing. When the en­gine is un­der min­i­mum load (the ve­hi­cle is trick­ling along in slow-mov­ing traf­fic) and the turbo isn’t do­ing any­thing (as it re­lies on the en­gine’s ex­haust pres­sure to gen­er­ate boost) you crank up the en­gine’s com­pres­sion ra­tio to com­pen­sate.

Con­versely, when the en­gine is un­der high loads (out on the high­way pulling up a steep hill, for ex­am­ple) you drop the com­pres­sion ra­tio right down and let the turbo go to town.

In­finiti claims that its VCR en­gine can vary the com­pres­sion ra­tio seam­lessly from a diesel-like high of 14:1 to a very mild 8:1. The over­all con­trol of the com­pres­sion ra­tio, the turbo’s boost pres­sure, ig­ni­tion tim­ing, valve tim­ing, etc. is done elec­tron­i­cally and is all in­ter­linked. The vari­able in­take­valve tim­ing also means the en­gine can switch to an Atkin­son cy­cle mode (see op­po­site), which is all part of the en­gine’s adapt­abil­ity.

In­finiti’s VCR en­gine changes the com­pres­sion ra­tio by ad­just­ing the height the pis­ton reaches in the cylin­der, as shown in the ac­com­pa­ny­ing il­lus­tra­tion. It’s a bit un­clear ex­actly how all this works, but the key is the so-called ‘mul­ti­link’, which ap­pears to ad­just the crankpin off­set and hence the en­gine’s stroke. In­finiti prom­ises more will be re­vealed in the com­ing months.

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