When the internal combustion engine was invented, more than 100 years ago, the early pioneers were only too happy if the engine ran any old how. Later on, they expected their creations to hold together, and to last for a good number of years. After WW2, Japanese technology showed it was possible to design an engine that did not leak oil. More recently, engines were fitted with balance shafts to counter vibrations. A modern engine has to meet all expectations, while using as little fuel as possible, with super-clean exhaust emissions. Computer-controlled mixtures and ignition timing have removed most of the inefficiencies that used to plague the combustion process, and the emphasis is now shifting to other ways of improving efficiency.
Volumetric efficiency, defined as the amount of mixture actually inhaled, divided by the theoretical amount indicated by the piston displacement volume, has received a great deal of attention during the last 10 years. Employing variable length intake manifolds has improved breathing, but the paradoxical way an engine’s output is controlled is a big stumbling block. An engine is most efficient at full throttle, because there are few restrictions to airflow, and yet, when maximum power is not needed, a throttle butterfly is used to strangle the air intake. This reduces the volumetric efficiency during most of an engine’s working life, because very few engines are required to run at full throttle for any length of time. It’s very much like expecting an athlete to run with an adjustable clamp on his windpipe!
This means that for every driving situation a different size engine should be used, running at full throttle, in order to achieve maximum volumetric efficiency. This is not very practical, although a six-cylinder engine running on anything between one and all cylinders, depending on the amount of power needed, would be an interesting experiment. Mercedes-Benz, and one or two other manufacturers, offer a cylinder cut-out on some of their V8s, which use only four cylinders when cruising. Another approach is to use all the cylinders, but make them more efficient by varying the compression ratio while the engine is running. It’s amazing how many schemes have been tried or patented.
Varying the compression ratio also takes care of the danger of non-uniform combustion occurring at maximum cylinder filling. This is called detonation, and it is normally caused by either a high compression ratio (CR) or a too-generous spark advance, or both, for the type of fuel being used. In older engines this meant that the recommended spark advance had to be kept conservative, because some of the engines coming off the assembly line would have had one or more cylinders with a higher CR than the rest, due to manufacturing tolerances. In modern engines, a knock sensor allows the electronic control unit to optimise the ignition timing per cylinder, so that a slight variation in compression ratio can be catered for. Hence, if the CR can also be varied, this must further improve efficiency.
Interestingly, the best value for the CR, in both petrol and diesel engines, from a frictional loss point of view, is about 12:1. Above this value, the ring friction, caused by increasing pressure behind the rings and not the side thrust, becomes excessive. This is also a limiting factor for petrol engines as far as detonation is concerned, because above this value detonation becomes increasingly likely. Diesel engines can go as high as 22:1, because detonation is not a problem due to the excess air that is always present, and they need the high compression heat to start easily, as their fuel is less volatile. This ratio may come down in the future, to reduce the frictional losses. Some big industrial diesels run on a 12:1 CR, but their fuel has to be specially heated.
The compression ratio is actually a ratio of volumes, and is calculated by dividing the volume above the piston at bottom dead centre by the volume above the piston at top dead centre. This translates into CR = volume swept by the piston plus the volume of the combustion space, divided by the volume of the combustion space. This shows that the compression ratio can be changed by either changing the volume of the combustion space or the volume swept by the piston. The latter requirement effectively reduces to changing the stroke length, because any change to the bore diameter while running is difficult to visualise.
Changing the combustion chamber volume
This is not a new idea. Single-cylinder experimental engines supplied by Ricardo Ltd to oil companies, universities and other research establishments since the late ’20s have incorporated a cylinder barrel that can slide up or down in the crankcase by means of a toothed rack on the outside, thus changing the combustion space volume. The rack and barrel is moved by turning a crank-handle geared to it, and there is a scale on the outside to calculate the compression ratio. This can be done while the engine is running, to study detonation and fuel quality, for example. The valves are operated by a single overhead camshaft, driven by a vertical shaft that incorporates a splined coupling, so that it can change length.
The best-known modern example of such a scheme is the Saab moving head concept, shown to the press in May 2000. The design calls for a line of pivot points in the block, and the engine is designed so that the cylinder head, barrels and everything above it can swivel by up to four degrees. This means that the top- and bottom-dead-centres of the piston motion change, so that the combustion volume is altered. The four degrees swivel is enough to reduce the nominal CR from 14:1 during cruising to 8:1 when maximum power is needed. The nominal CR can be high during part-throttle operation because the actual pressures are lower, due to the semi-closed throttle butterfly position, but the CR is lowered at near-full-throttle because Saab uses this system with a supercharger.
This particular combination of variable CR and a blower means that a high compression small engine is available for cruising economy while a lower CR, boosted engine is on tap for maximum power. The Saab engine is a five-cylinder 1,6 litre unit that produces the sort of power one would expect from a 3,0-litre.
Another scheme being revived is to change the combustion chamber volume by moving the crankshaft up or down. Some fuel research engines have been built with this arrangement, but there is also a serious attempt by FEV Motorentechnik, an engine research company with branches in Germany and the US, to promote such an engine, in turbocharged form. The main-bearing housings incorporate eccentric discs that can be rotated by a gearwheel attached to a shaft driven by an electric motor. These discs contain the main bearing shells, and as they rotate the crank is lowered or raised by up to four millimetres, thus changing the CR to any value between 8,5:1 and 16:1. The CPU controls the rotation, and hence the CR, after receiving input from sensors about intake air temperature, mass airflow, ignition timing, mixture strength and turbo boost. One of the disadvantages is that the engine needs special couplings at the flywheel end and the accessory drive end, because these drives are obviously not able to adapt to crankshaft movement.
The prototype turbocharged FEV engine was installed in a 1,8-litre Audi A6 and shown to the press at a recent SAE World Congress for comparison with a standard 3,0-litre A6. The FEV car went from 0 to 96 km/h in 7,39 seconds, 1,1 second faster than the 3,0-litre, and consumption averaged 7,7 litres/100 km against the bigger car’s 10,5 litres/ 100 km over a set route. The end result seems to very similar to what Saab has achieved. The chamber volume can also be varied directly, by means of a moveable piston or valve. Ford has patented a cylinder head that incorporates a small valve in the centre of the head, whose opening will reduce the CR. It was invented as a way to alleviate detonation, but could easily be modified to serve as a variable CR device. The piston is spring-loaded and is activated by a cam whose movement is controlled electronically.
The Alvar/Volvo engine design uses small inverted pistons whose bottoms form a part of the combustion space, controlled by tiny conrods on an overhead shaft, to change the combustion volume in a precisely-timed way. Both patents are of little practical value because they affect the combustion camber shape adversely, and would most likely be affected by carbon build-up. Designers have also been looking at ways to change the piston deck height, and in this way change the combustion chamber volume. A Ford patent shows a sliding piston top whose clearance above the lower portion is controlled by the difference between the force delivered by a Belleville washer (dished washer) and the force of combustion. As soon as the combustion pressure reaches the spring pressure, the washer should collapse, thus lowering the CR and reducing the chance of detonation. This would be its only purpose, so that it is not supposed to function as a controlled variable device. Mercedes-Benz has been working on a hydraulically changed piston top height, by using oil under variable pressure to raise or lower a moveable piston top. Both these schemes would make the piston heavier, while the ideal is to make pistons lighter, to reduce load on the bearings.
Another way to raise the position of the top dead centre while the engine is running is to have a variable-length conrod. This seems a bit far-fetched, but engineers at the University of Applied Sciences in Hamburg don’t think so. They’re working on a special two-piece conrod that can be lengthened or shortened by means of hydraulic pressure, when required. If they can come up with a workable scheme, it will at least have the merit of being mechanically simple. It’s interesting to note that on a Formula One engine, the stresses at 18 000 r/min are sufficient to lengthen the conrod by an amount that will raise the CR noticeably.
Conrod linkages
A number of designs use linkages that affect the conrod motion. The Nissan scheme (see figure), uses a small conrod, mounted on an eccentric, to change the main conrod orientation via a link in such a way that the position of top dead centre is changed. Similar schemes have been patented by Peugeot, Ford and FEV. A far more exciting proposal is the Mayflower engine concept, proposed by Dr Joe Ehrlich, whose name is well known to older followers of motorcycle racing. During the ’50s and ’60s, he designed some very fast racing two-strokes under the EMC name. His engine uses a sliding link, whose pivot can move in two planes, to change the way the con-rod moves. This makes it possible to change the compression ratio by means of a TDC change, and the capacity by means of a stroke length change. It can, for example, have a short intake stroke and a long exhaust stroke.
Time will tell which of these schemes will eventually win commercial acceptance. Most of them can incorporate downsizing, which is the automotive buzz-word at the moment, but the Mayflower seems particularly clever, because it can vary both CR and displacement. But one should bear in mind that it is, once again, not a new idea, and one has to ask why previous ones failed. On the other hand, a lot of promising designs have failed because they were too early, before the electronic age, and the variables could not be controlled easily.