Nissan readies ultra-low CO2 petrol engine for Micra
Fuel injection, supercharger make for zero road tax
Nissan's presence at next month's Geneva Motor Show may be dominated by its Leaf e-car and ESFlow concept e-sportster, but it will also be unwrapping a petrol engine it claims will deliver the world's lowest carbon output.
Dubbed the 'DIG-S', the engine, which is being designed for the Micra, pumps out 95g of CO2 for every kilometer driven.
That's low enough to count for no Vehicle Excise Duty (VED).
The moniker stands for Direct Injection Gasoline Supercharger. Each of its three cylinders run a longer the fuel intake stroke than a regular petrol engine has - an approach engineers call the 'Miller Cycle'. The supercharger compresses the air fed into the engine to counter the loss of air back up the pipe that would otherwise result from holding the intake valve open for longer.
The upshot of all this is a much higher compression ratio which, in turn, ups the engine's combustion efficiency, resulting in that low CO2 output figure.
Nissan said the DIG-S engine produces 72kW (96.6 horsepower) and 142Nm of torque. That, the company claimed, gives the 1.2-litre engine the same power output as a conventional 1.5-litre four-cylinder job.
Replace the manual gearbox with a CVT, and the CO2 output goes up from 95g/km to 115g/km, taking the road tax from zero to £30.
Getting 95g/km will depend on your driving style, but that's fortunately not yet a consideration for VED calculations.
The Micra DIG-S is expected in 2012. ®
Wait until all cars qualify...
When all modern car qualify for zero rated road tax, expect the government to move the goal posts and expect to start paying road tax again...
Continuously Variable Transmission.
They've been on things like Scooters for ages, but they are beginning to be seen on small cars as well, they normally work by some variation of two cones pointing opposite directions with a rubber band connecting the cones with one cone attached to the engine, and the other cone attached to the power shaft, moving where the band connects with the two cones changes the ratio between them and thus the gear ratio, but with out relying on a limited number predefined gear ratios like in a gear box.
Upside, you can keep the engine revs in a much narrower band and you don't need a clutch which makes for much smoother accerlation.
Downside is they are all based on friction between 2 surfaces rather than mechanically interlocking teeth like you get in a gearbox which means without a huge surface area to provide the friction there is a limit to the amount of torque you can push through it.
Who's copying whom?
The 'Atkinson' engines used in the Prius aren't real Atkinsons, but Otto cycle engines, modified to use a Miller combustion cycle.
James Atkinson gave his name to a combustion/valve-opening cycle he developed in the late 19th century, having observed and talked to the valve-operators, on the four stroke engines used in pumping out mine water in the later Victorian period. These were fueled by stuff like finely-ground coal dust, blown into the combustion chamber using belllows, and had a very slow rotation rates. It had long been known that a perfect rhythm, in hauling on the valve levers, to open and close them, would lead to the best power output from such engines, and a good valve operator (who knew the rhythm) was a highly prized man.
Atkinson's interest was spurred by this, and he went on to study the effect, described the maths involved, in an idealised valve-overlap system, for a four stroke engine back in 1882: the Atkinson cycle. So good was this work, that the effects, first seen in engines with rotations as low as 1 rpm, still hold true for modern four strokes, of 18,000 rpm.
In order to evade certain patents, held over the Otto cycle engine, however, Atkinson used an ingenious tripple crank mechanism to achieve just one turn of the primary drive crank, per four strokes of the engine. It was totally superfluous to the actual effect, but did get around getting sued by the patent holders. This is why the 'Atkinson' engines in modern cars are not true Atkinsons, but Ottos, that use the Atkinson cycle.
Ralph Miller's idea, of 1940, was to supercharge the inlet of an Otto engine, running an Atkinson cycle, overcoming the low power-density of the Atkinson engine. His engines were intended for Marine and static power-generation use, and were heavily patented. This led to the automotive industry largely disregarding this type of engine for road use - just as Rudolph Diesel's successful patenting of the compression-ignition engine, meant that the less efficient carburated Otto engines were used in most car and motorcycle engines for much of the last century.
So there you go: if you like (and many of you will, of course) we can conclude that patents are so evil, they actually lead to global warming (unless your names happen to be 'Lewis Page' or 'Andrew Orlowski', of course - in which case global warming is caused by Stephen Fry).