The World's-First Variable Compression Ratio Engine Could Kill Diesel Forever
A gasoline piston engine that can dynamically change its compression ratio óthat is, the amount by which the piston squeezes the fuel-air mixture in the cylinderóhas long been a holy grail of engine design. It looks like Nissan is the Indiana Jones of engine design, then, because its luxury division Infiniti will introduce the first production variable compression ratio engine in the 2018 Infiniti QX50. Could this be the death knell for diesel?
The engine is a 2.0 liter turbocharged four-cylinder that Nissan is calling the VC-T(Variable Compression-Turbocharged). The VC-T engine will allow for the compression ratio to be dynamically varied from between 8:1 all the way up to 14:1. Nissanís press release describes this as:
"The ingenuity of VC-T engine technology lies in its ability to transform itself and seamlessly raise or lower the height the pistons reach. As a consequence, the displacement of the engine changes and the compression ratio can vary anywhere between 8:1 (for high performance) and 14:1 (for high efficiency). The sophisticated engine control logic automatically applies the optimum ratio, depending on what the driving situation demands."
... though, really, itís actually a bit more complex than that. Low compression isnít really ďhigh performance.Ē Racing engines tend to be very high compression, with NASCAR engines, for example, having a compression ratio of 12:1. High compression is also more efficient, so it would seem reasonable to ask why not just make engines that are constantly at high compression?
The reason is that high compression also causes knock, or unwanted pre-detonation of fuel. A diesel uses compression only to ignite the fuel/air mixture, and the same thing can happen with a gasoline fuel/air mixture at high compression levels, causing detonation at the wrong times. So, the variable compression ratio engine can drop the ratio during periods of high turbocharger boost to keep knock from happening, while increasing the ratio during periods of lower boost, for greater efficiency. This gives both more power when needed, and better economy when not.
Also, the engine is equipped with variable valve-timing technology, allowing the engine to operate like an Atkinson-cycle engine during low-demand driving. Atkinson cycle engines hold the intake valves open a bit longer to let some air escape and lower the effective compression ratio as well, for better efficiency. Theyíre usually found in hybrids, where the electric motors can compensate a bit for the lack of oomph.
The result of all this is an engine that gets 27 percent better fuel economy than Nissanís 3.5-liter V6, at roughly the same HP and torque. According to Reuters, at a press conference Nissan engineers said the new engine matches turbodiesels in torque, and is actually cheaper to build than modern turbodiesel engines.
Thatís why engines like this have the potential to replace diesels in many contexts, possibly (and Iím speculating here) including eventual lower-cost versions for countries where diesel is the primary fuel like India. Perhaps this could be in the form of a variable-compression two-cylinder gas engine designed for small passenger cars?
The flexibility of the engine seems to be very impressive. It manages to do the variable-compression trick via, according to their patent and Infiniti drawings, via a sort of multi-link piston rod. The piston rod has a central pivoting multi-link that can change its angle, which in turn changes the effective length of the piston rod, which changes the amount the piston moves in the cylinder, which, finally, changes the compression ratio.
It seems to be a very clever system, though it does seem dramatically more complex than a single, dumb, forged piston rod. That may be why Nissan says the engine makes sense as a four and not a V6 or V8; the cost of all these articulated piston rods may be just too high. Iím guessing, but that seems plausible.
In a hybrid context, an engine like this would be hard to beat in terms of across-the-board power and efficiency, thanks to its ability to dynamically adjust how it behaves. It could be a relatively high-compression Atkinson engine supplemented with electric motors, all the way to a lower-compression but very highly boosted performance engine, all in the same car.
The engine will be officially revealed on September 29 at the Paris Motor Show.
I wonder how this would translate to octane requirements/recommendations at the pump...
Funny I just read about this earlier. Just when you think the internal combustion engine has peaked, this comes along. Clever stuff indeed, no doubt we'll see it in future Renault models too as they are the same company now. I'd be super sceptical about economy figures though, this clever engine tech can often promise so much and deliver very little, but time will tell. I can't imagine it would come close to a diesel for fuel economy.
They say it costs less than a diesel engine, but it obviously cost more to develop and test it. There may be an initial higher cost to cover recalls, insurances, warranty and maybe the odd premature failures. New tech always costs more, but drops significantly in time.
I'm waiting until this is in production and real number and experiences have logged plenty of miles before I get excited.
A gasoline (petrol) engine works on an entirely different principle than diesel. A big difference is that a gasoline engine introduces a calculated fuel-air mix into a cylinder, compresses it (at a fixed or now variable compression ratio), and thenvignites the fuel/air mix using a spark plug. It must always be sure to avoid pre-ignition, or detonation, or risk damaging the engine. A diesel compresses only air at a very high ratio, and to such a temperature that when the injector introduces fuel, it instantly ignites in the hot cylinder (assuming the engine has warmed up and the glow plugs are off).
With today's computers, the precise injection of diesel fuel can be varied not only in volume, but in time relative to Top Dead Center (TDC), as well as with multiple pulses at various times throughout the detonation cycle. This gives diesel engines a lot of flexibility in this area, which gasoline engines simply don't have.
The other factor is the price of the fuel. Where I live, diesel has been cheaper than regular gas, ever since I got my car. Will the variable-compression engine run on regular, or likely need the much more costly premium gas, which has a higher octane rating to prevent detonation at higher loads and higher compression ratios? MPG is not the only factor. "Cost Per Mile," the more useful metric, takes the fuel price into consideration as well as he fuel economy.
As I said, I'm not pooh-poohing this new engine. I'm just regarding it with some cautious skepticism before I conclude that it's the new savior.
2015 Audi Q5 "Progressiv" + S-Line + Scuba Blue, 3.0L V6 TDI
(Highest fuel economy for all Audi Q5s on Fuelly!)
I think Saab's system actually lifted and lowered the head block to vary the compression.
If the engine is direct injected, regular octane will be fine, but it could work with port injection. With VVT, the fuel charge will never see the 14:1 compression pressures for the efficiency mode; it is using an Atkinson cycle. The Prius has a 13:1 ratio with port injection. The intake valves close late during the compression stroke. The fuel charge is seeing a 10:1 CR at most. The higher compression ratio allows for a longer power stroke to extract more energy from the burning fuel.
That 14:1 compression ratio should really be called an expansion ration.
The question I have is what this system adds that a full range, Otto to Atkinson, VVT doesn't. Such valve timing systems are starting to come to market too.