That’s why Renault shocked the world earlier this month, when they unveiled a two stroke diesel engine(pictured above,) ostensibly for possible future automotive applications, as part of a slew of experimental technologies they are presently working on. The French auto giant claimed to be developing the engine with a number of partners across Europe for possible use in subcompacts and micro-cars. This move left a lot of people wondering, could the rugged two-stroke diesel one day make its way into our cars?
The question is an interesting one to explore. Presently, the two stroke diesel is the engine of choice for large ocean going vessels, supertankers, and container ships. Many diesel-electric power-plants found in trains also employ engines of this configuration, which charge batteries, funneling power to the wheels at necessary output via an electric motor. However, despite all of the benefits of two-stroke engines, there are several consequential drawbacks which have largely precluded their entry into cars in recent years. Of course, the traditional two stroke engine is wrought with pollution problems. This stems largely from the fact that oil needs to be mixed with the fuel mixture. The engine not only has a higher percentage of gasoline/diesel fuel going uncombusted(a primary culprit of carbon monoxide and other pollutants,) but also produces way more pollutants due to byproducts of burning oil. Problems with unburnt fuel escaping out the exhaust are also encountered, especially on more traditional designs where the cylinder also serves as the valve-train.
There is also a reason why you see this engine employed on large ships and trains. Though the two-stroke diesel produces loads of power and lots of torque, the engine’s power band is quite narrow compared to other engines. Of course, this doesn’t matter on a large ship that runs at the same speed for days on end, or on a train that only uses the engine to power batteries and a generator. However, on a city car that has a need for operation across a wider rpm range, this can be problematic.
But, despite all of the drawbacks that have prevented the two-stroke diesel from ever residing under the hoods of our daily drivers, it does have a few advantages over its four stroke cousin. In theory, a two stroke can produce twice the power of a four stroke, due to the fact that a given cylinder’s power stoke happens once every rotation of the crankshaft, instead of once every other, as in a four stroke; hence, a two-stroke can produce much more power, proportionally, from a smaller displacement. The two stroke’s simplicity also becomes an advantage. Due to the fact that a two-stroke has less moving parts, a much higher thermal efficiency can be realized over a four stroke. We’re talking about a 50% thermal efficiency for a two-stroke diesel, compared to only 35% for a four stroke diesel. Coupled with the greater efficiency of a diesel engine, this combination could yield promise in a world where efficiency and fuel economy mean everything. Weight savings from the lighter design are obvious here, as are lower manufacturing costs.
Though the prospect of the two-stroke diesel going into road cars has heretofore been bleak, there has to be at least some promise in it if Renault is investing. Renault’s two-stroke diesel is a two-cylinder 730 cc(0.73 L) called the “Powerful,” that produces around 67 horsepower and 107 ft. lbs. of torque from 1,500 rpm. The engine uses the relatively new twin-charging technology being employed increasingly in today’s automotive market to boost power. This setup uses a turbocharger to boost output under the engine’s normal operating range, while a supercharger, which may or may not be electrically driven, spools the compressor at lower outputs, where the turbocharger produces insufficient boost; in other words, you reap the benefits of both technologies.
Of course, any two-stroke diesel that went into a car would also have a closed oil system, thus eliminating emissions concerns arising from this. Contrary to popular belief, a two-stroke is not required to have an open oil system that necessitates mixture of fuel and oil. This is only warranted on small engines where it is not cost/space effective to have a built in lubrication system. Most small engines have conventionally been two-strokes, ergo the assumption that two-strokes can’t have self-contained oiling systems. It is true that the open crankcase is needed on a two stroke engine to properly pressurize the intake and force air into the cylinder. However, two-stroke diesels used in commercial applications get away with employing a closed crank-case with a self contained lubrication system, by making use of a supercharger or other scavenging means to force sufficient air into the cylinder. In many cases, more archaic Roots-style rotating lobe blowers have been used to accomplish this.
Renault’s engine employs the self-contained oil system, and other measures appear to have been taken to adapt the two-stroke diesel cycle for smaller, road-going applications. One of the main issues in need of correction is the power delivery across a wider power band. The twin-charging setup will undoubtedly increase engine output, and supercharging, in contrast to the turbocharging used on many diesels, will allow better performance in lower ranges. It is likely that modern high-tech supercharging technologies will provide a marked improvement over traditional Roots-style supercharging or exhaust based pressurization methods in terms of boosting engine performance across a wider range.. Renault’s engine also does not make use of the traditional ‘porting’ system where the cylinder functions as the valve-train. In fact, most commercial two-stroke diesel engines use poppet valves for the exhaust anyway and only maintain the traditional setup for the intake. However, Renault has taken this one step further, employing the use of poppet valves for the intake as well. This was also done by Briggs & Stratton on their one cylinder two-stroke engine built for their generators and lawnmowers. Merely imagine the intake valve opening as the cylinder approaches bottom dead center, and the exhaust valve opening as the piston approaches top dead center. A complex valve scavenging system is employed on Renault’s engine in order to keep gas flow and direction in check. Renault is also claiming CO2 and other emissions figures to be comparable, if not lower than, other engines of the same size.
The Renault engine ostensibly employs four valves per cylinder and variable valve timing. These advances, coupled with a high-pressure common rail system, injecting fuel at 2,000 bar(~29,000 psi,) help to drastically improve engine performance. This conceivably allows the engine to perform better and widens the power band on the engine significantly, perhaps to the point of being very capable for city driving. The Powerful also appears to be built specifically for coupling to a Continuously Variable Transmission(CVT.) For those of you unfamiliar with CVTs(pictured below,) they use one of several methods, the most common of which being adjustable cones oppositely adjoined, connected by a belt or chain, to give virtually unlimited gear ratios. In addition, they allow for one continuous shift, rather than separated, episodic gears. The abundance of gear ratios allows for an engine to more adequately operate within its optimal fuel efficiency range or its power band. CVTs also realize a much greater fuel efficiency over their manual or automatic counterparts, owing to the fact that gear ratio can be adjusted instantaneously to the most efficient ratio at a given output.
In addition to achieving a much higher thermal efficiency over the four-stroke diesel counterparts and building an engine which is much lighter, the Renault team claims to have realized a power to weight ratio that is 1.7 times that of comparable four stroke engines. Naturally, however, Renault and their partners aren’t satisfied until that number is closer to the theoretical 2.0 possible in this scenario. The French automaker also wishes to raise the power output of the engine a few more horsepower until it goes into any road-cars. Though 67 horsepower would probably be enough to power a city car or subcompact like the Twingo(pictured below,) the Powerful could benefit from another 10-20 horsepower to put it in the beyond safe range.
Though many people probably can’t fathom the prospect of a two-stroke diesel of all things going into cars, it may happen in the coming years. Renault plans on developing the Powerful further before making any announcements on the engine’s future. A start-up out of the American Northeast also aspired to market two-stroke opposed piston-opposed cylinder diesel engines and had reportedly drawn investment from Bill Gates, but this effort seems to have gone dormant some time around 2011. If one thing is clear, it’s that the Green Revolution will leave no stone unturned in the quest to reap higher and higher fuel efficiency and emissions numbers.