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Engine Trends

The infernal combustion engine has been with us for a long time - since about 1885. Familiar layouts soon appeared with, for example, six cylinders seen as early as 1902. Multiple valves per cylinder, double overhead cam-shafts, super-chargers, turbo-chargers, and fuel injection are all well and truly pre-war. After that, it might be said that there was no very novel and lasting engine concept (perhaps the Wankel rotary engine excepted) until the oil crisis and stricter anti-pollution laws started a movement towards greater engine efficiency. Engines have grown more efficient and less polluting (if well maintained) since the 1980s but cars and 4WDs grew heavier and bigger in the 1990s as oil prices fell, cancelling out some of the gains. Some day oil will become inordinately expensive to use as a fuel, as OPEC's production squeeze of 2000 reminds us. Getting ready for that day, a number of refinements of, and alternatives to, the internal combustion engine are coming to show-rooms or running in laboratories and in test vehicles. - 4wd.sofcom.com/4WD.html, 4/2000

Direct Injection

Direct injection is where fuel is injected (directly) into the cylinders, not mixed with air in the inlet manifold or inlet ports before being drawn into the cylinders. Diesel engines inject fuel either into the cylinder proper or into a small chamber off the cylinder, but direct injection on petrol (gasoline) engines is not new either, e.g. the world war II DB601 aircraft engine (?Me109?) used it [2]. However, petrol direct injection is a recent development in mass produced cars. Mitsubishi released such a system in the 1996 Galant [2] and the 2000 Pajero sold in Japan uses it. (c) 4wd.sofcom.com --> The Australian Orbital Engine Company started work with Mercedes Benz and Siemens in 1998 on a direct injection system for cars that is said to use lower pressures than Mitsubishi's. (Orbital's direct injection is already used in Mercury Marine 2-stroke outboard engines.)

The advantages of direct injection are that the fuel can be placed in the combustion space in a more controlled manner than with conventional (inlet) injection systems. "Lean running" is possible by forming a richer cloud in the vicinity of the spark plug. The challenge is to control precisely the amount, size and distribution of the fuel droplets to suit varied driving conditions, and to do this reliably over the life of the vehicle. Lean mixtures also tend to lead to the formation of oxides of Nitrogen, NOx, a pollutant limited by legislation although exhaust gas recirculation can be used to reduce engine temperatures and thus NOx formation to some extent.

Electric Propulsion

Electric vehicles are nothing new, many being used in the early 1900s. However batteries remain large, heavy, expensive, slow to recharge and have a limited life. Hybrid systems (below) show much promise, allowing a small efficient petrol engine to carry the "base load" with batteries to cover high power demand.

As of 2000, the best bet for electric power seems to be the fuel cell.

Fuel Cells

If an electric current is passed through water, the water is split into hydrogen and oxygen. The fuel cell, as used in space-craft, reverses this reaction combining hydrogen and oxygen to release electrical energy with pure water as a by product. In principal hydrogen could replace petrol and diesel as the fuel for transport. In the short to medium term hydrogen could be produced from hydrocarbons, in the long term power stations, perhaps solar powered, could produce it. The attraction of using the fuel cell to generate electricity, over burning the hydrogen in an internal combustion engine, is that the fuel cell is very efficient indeed, achieving 45% to 60% efficiency (c) 4wd.sofcom.com --> versus a petrol engine's 15% to 35% [1]. There are problems: hydrogen is an explosive gas that is difficult to store. This could be solved in the "cost no object" space program, but distributing hydrogen, storing it at service stations, refuelling cars safely, devising fuel tanks that are safe-ish in accidents is going to be difficult. However, experimental cars such as the Daimler Chrysler Necar 4 have been demonstrated using hydrogen fuel cells.

As an interim measure, systems that generate their hydrogen on demand from a liquid hydrocarbon, most likely petrol (gasoline) or methanol, may be adopted. The fuel can be distributed using existing infrastructure. However efficiency falls because of the conversion process, carbon dioxide is a by-product (although CO2 is also produced if fossil-fuel power stations are used to generate hydrogen), and sulphur can poison the fuel cell if it get past the hydrogen generator.

2000 November: See the Mercedes Benz Necar 5.

Hybrid Engines

A hybrid propulsion system uses a petrol or diesel engine with an electric motor in some combination. One variation is to have the wheels driven only by the electric motor or motors, current coming from batteries. The petrol engine (say) drives a generator to charge the batteries; it can be turned on and off as needed, and can be optimized to run efficiently in a narrow rev' range. Batteries can be smaller than in an all-electric car because they only have to supply current for short periods. The hybrid car still has the range of a conventional petrol or diesel car.

A second variation is to have a relatively small petrol engine drive the wheels through a mechanical transmission. An electric motor provides assistance when high power is needed - overtaking and climbing hills. Some engine power is diverted to charging the batteries at times of low power demand. The Honda Insight is one such car, for sale in America from December 1999 (probably being sold at a loss to test the waters). It is said to achieve a low consumption figure of 3.85 litres per 100km.

The electric motor can also act as a generator slowing the car - called regenerative braking - to help recharge the batteries and reducing wear on the brakes.

A hybrid car can use a smaller internal combustion engine which spends most of its time operating in the more efficient part of its range. (c) 4wd.sofcom.com --> The next "Jeep", the replacement for the Hummer, the RST-V is planned to to employ hybrid propulsion for the purposes of quiet running and limp-home redundancy.

2000 November: ChryslerDaimler announced plans to offer a hybrid system in the Dodge Durango 4x4 in 2003.
Also see the Toyota Prius.

Miller Cycle

As discussed under super-charging below, the efficiency of an engine is generally improved if its expansion ratio is increased, i.e. if as much energy as possible is extracted from the exhaust gases so that they leave the tail-pipe cold and at slow speed. This could be achieved by having an exhaust stroke that is longer than the compression stroke. At first sight this seems to be a geometric impossibility, but it can be managed by sacrificing some of the upward movement of the piston - leaving the inlet valves open for a while. Some of the inlet gases will be expelled back into the inlet manifold, but so what? The compression stroke uses a fraction of the upward movement of the piston, but the power expansion stroke uses its full downward movement. Such tricks have been played with large marine diesel engines for many years, but they only recently became practical on car engines with improved tolerances and electronic controls. The idea was patented by Ralph Miller in the 1940s.

Mazda has been fitting 2.3 litre Miller cycle engine to its Eunos 800M since about 1997. The Mazda engine incorporates a super-charger and inter-cooler, the explanation being that this compensates for the effective reduction in engine capacity due to the shorter compression stroke; the super-charger is said to be more efficient at compressing the intake mixture than a full compression stroke would be. (The logical extreme would seem to be to do all the compression with a super-charger, closing the inlet valves just before igniting the mixture, but perhaps super-chargers do not work well at such high pressures?)

  • Mazda Miller-cycle, 4-valves/cyl, V6 DOHC
  • bore: 80.3mm, stroke: 74.2mm, swept volume 2254cc
  • power: 149kW at 5500rpm, torque: 282Nm at 4000rpm

-

Steam

There is no immediate prospect of steam power making a come back on the road; steam is not the victim of a conspiracy theory, the thermodynamics are just not right. The most recent Australian link with automotive steam seems to be the various experimental Pritchard steam cars built during the 1960s and 1970s, and the Gvang prototype of 1972 (Davis 1987).

However as of 2000, a British team [www] is looking at raising the land speed record for a steam powered car. A Stanley steamer set a world land speed record of 127.66mph at Ormond Beach (Daytona) Florida in 1906. This stood until a petrol engined Benz achieved 131mph in 1910. The British team includes Glynne Bowsher who was the chief mechanical designer for the supersonic "car" ThrustSSC. "The initial target is 150mph but the car is being designed for speeds of 200 mph or more."

Super- and Turbo-Charging

Forcing more air into a cylinder allows more fuel to be burned, generating more power from an engine of a given weight and size; that's the basic idea behind super-charging and turbo-charging. A super-charger is driven by the engine, either by gears or by a belt from the crank-shaft. Super-charging was popular in 1930s' racing engines - Bentley, Auto-Unions and Mercedes Benz. The latter has revived it in a line of Kompressor models in the 1990s. Even GM Holden has bolted a super-charger to the Commodore V6.

Super-charging increases power but not necessarily efficiency; it increases the compression of the intake air but not its expansion after the fuel is burned. Think of a sealed cylinder at room temperature and pressure with the piston at bottom dead centre. As the piston moves up, doing work, the air is compressed and grows hotter. As the piston passes top dead centre it is forced down by the compressed air which grows colder returning to room temperature and pressure. If there were no frictional losses and if the cylinder were perfectly insulated there would be no nett loss of energy. In a working engine, fuel is ignited near top dead centre. This makes the air-fuel mixture hotter than ever, which increases the pressure even more, and drives the piston down strongly. Now when the piston nears the bottom the gases are still relatively hot and under some pressure, i.e. they contain residual energy which is simply dumped into the exhaust system and wasted. Having said that, if the super-charger is used intermittently for high power demand only, it can allow the use of a smaller, lighter engine; see variable compression below.

An engine's efficiency can be improved if its expansion ratio is increased. In principle, this could be done by feeding the exhaust gases into a second (larger) low-pressure cylinder - forming a compound engine - but the extra weight, size and complexity make this impractical in a car engine (but note that double and triple expansion designs were common in large steam engines). A common alternative is to drive a turbine from the exhaust gases. Invariably (?) the turbine is used to drive a compressor, giving us a turbo-charged engine. By a happy accident, the turbine extracts most power at wide throttle openings just when high boost is wanted. The modern turbo-charging craze, arguably begun by Audi, has generally been promoted as increasing power rather than efficiency.

No law says that the turbine must drive a compressor. It could assist in driving the crankshaft and such designs, also called compound engines, did appear in aircraft piston engines: A super-charger was driven from the crankshaft and an exhaust-gas turbine helped the pistons to turn the crankshaft and thus the propeller and, of course, the super-charger. Some "eccentrics" have recently rediscovered the logical progression to the gas turbine and jet engine: Remove the role of the reciprocating piston, replacing it with a fixed combustion chamber. Apparently it is possible to make a crude gas turbine from an automotive turbo-charger although extracting any useful power from it is another matter. Do not try this at home - there is no little danger of hot, sharp bits of metal escaping at high speed. Some car makers such as Rover did experiment with gas turbines but the poor fuel economy of car-sized units was the main stumbling block.

Two-Strokes

The 4-stroke engine has effectively pushed the 2-stroke engine aside in petrol (gasoline) driven cars. The 2-stroke gives twice as many bangs per revolution - more power for less weight - but pollution is a problem with some fuel and lubricating oil escaping into the exhaust unless direct injection is adopted together with a sealed sump. The Orbital Engine Company has put their direct injection system into 2-stroke outboard motors for boats.

On the other hand, the 2-stroke diesel engine is alive and well - in some trucks. A 2-stroke needs the intake air to be lightly pressurized to blow out the last exhaust gases from the cylinder and a super-charger can manage this quite well. Diesels inject fuel into the cylinder near top dead centre when all the valves are closed. One wonders if a 2-stroke petrol engine with a super-charger and direct injection might be a goer.

Valve Operation

Inlet and outlet valves in all (?) production engines are operated by camshafts - either on the side of the block acting via pushrods and rocker arms, or single (sohc) or double (dohc) overhead camshafts. Audi has been leading the pack towards no less than five valves per cylinder - three inlet and two outlet valves (although F1 and sports-car racing engines remain on four per cylinder). Toyota seems to have been the first to offer 4 valves per cylinder in a passenger diesel 4WD.

High power is developed at high rev's and requires inlet valves, in particular, to open wider and for longer. Engines with these characteristics typically develop maximum torque at high rev's and can be inflexible and difficult to drive. Various systems became available from manufacturers such as Honda from the 1980s to vary (a) the point at which valves open and (b) the duration of the valves being open; typically extra lobes on a camshaft can be moved together or apart to cover a variable angle. Such an engine can be both tractable at low speed in traffic and sporty when given its head.

Springs, to close the valves, have been with us since the year dot. Some racing engines have had desmodromic valve systems - valves opened and closed by cams. Since the 1990s+/-  formula one engines and some experimental engines have used compressed gas instead of valve springs - it must be lighter and faster acting. We probably will not see this on production cars.

In the late 1990s, Mercedes Benz has been experimenting with electrically operated valves, i.e. opened and closed by solenoids. The advantage is completely variable valve timing without any mechanical complexity. In fact it has been possible to do away with the throttle, controlling the amount of air entering the cylinders through the valve timing. There must be questions over reliability.

Variable Compression

v c

2000 February: Saab showed a prototype variable compression engine at the Geneva Motor Show. This engine tries to solve the problem of being both a small, fuel-efficient engine and a large, powerful engine by changing its geometry.

Devised by Per Gillbrand 20 years ago, the Saab Variable Compression (SVC) engine can vary its compression ratio between 8.0:1, for wide throttle, high-power settings and 14.0:1, for light throttle, fuel efficient operation. The trick is made possible by tilting the "monohead" (head and cylinders) by up to 4° about an axis on the left of the engine (in the diagram). This changes the distance of the cylinder head from the crank by a few millimetres - enough to change the cylinder volume at top dead centre by nearly a factor of two.

By itself, lowering the compression ratio would not increase power at all but it enables the super-charger to switch to a higher boost pressure without any danger of pre-ignition. Squeezing more air into the cylinders allows more fuel to be burned - more power.

The 1.6 litre, 5-cylinder prototype delivers 168kW of power and 305Nm of torque. Boost pressure varies up to a maximum of 2.8 bar. In effect, the engine can behave like a 1.6-litre motor out of a shopping trolley or like a 3+ litre engine on demand. (e.g. The 3.2 litre V6 in the 1999 Mercedes ML320 produces 160kW.)

  • Saab Variable Compression engine, 1.598 litres, 5-cyl, c.r.: 8:1 to 14:1 variable,
  • Max. monohead tilt angle: 4 degrees, Max. super-charger boost: 2.8 bar
  • power: 168 kW, torque: 305 Nm, bore: 68mm, stroke: 88mm

- © L. A11ison

Go to the mechanical and 4wdonline pages

References

[1] A. J. Appleby. The electrochemical engine for vehicles. Scientific American, p58, July 1999.

[2] S. O'Ciardhubhain. An injection of petrol economy. The Engineer, p46, 30 November 1995.


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