The Diesel engine is the second according to its popularity what is used in modern automobiles. Some people say that it will be the basic engine for automobiles in the close future; some people say diesel engine is obsolete. Nevertheless diesel engines are continued to produce in million quantities all over the world and the approaching fuel crisis makes to solve faster the fueling problem for individual and public transport. We will estimate if the diesel engine would be the appropriate solution.
The first man, who had invented the engine with ignition from compression, was E. Steward. He was interested in engines, what can work without spark plugs. In Steward’s engine the air was compressed and compressed air was blown into the combustion chamber. Unfortunately, Steward had not come into mind to test the efficiency of that type of engines.
Developing the concept of “economy-type heat-engine”, Rudolph Diesel in 1890 invented the engine much more efficient due to high compression ratio. In his book he suggested to use the powdered coal, but it was difficult in real life – the coal dust has an abrasive properties and it should be found the way to put it somehow in combustion chamber. So it was suggested to use the tailing that remains after oil refining in such engines. So in 1897 Diesel had patented the engine design, later named Diesel engine.
It was excellent idea, but the fuel should be injected into cylinders and ordinary compressors were insufficient to inject oil residue and other tailing fuel. The heavyweight compressor, necessary to inject the fuel and low revolution rate had limited the use of such engines to applications, where heavy weight has low significance such as ground stations and ship engines.
In the 20-es of XX century it was serious approach to diesel engines. Robert Bosch improved the high-pressure fuel pump (hereinafter referred to as HPFP), that had resulted in possibility to increase the revolution rate and to avoid the use of bulky and noisy air compressor in Diesel engines. Since 1927 Bosch GmbH begin to put on market mechanical HPFP for diesel engines. But unfortunately, nobody had heard the prophet in the own country. Even on tanks the Germans had continued to mount fire-hazardous and wasteful petrol engines.
Only in the 50-s the diesel engine were started to mount into the trucks. After the fuel crisis in 1973 the car manufacturers also began to interest in diesel engines. Nowadays Europe leads in production and consumption of the diesel-powered automobiles. The Americans still can’t put out the thought of cheep petrol, so even on large pickup and off-road they continue to mount multi-liter monster-like petrol engines.
The design of Diesel engine is mostly alike the one of the petrol engine – both have pistons, cylinders, valves. But the ignition system in Diesel engines is lack in principle. Instead of the usual spark in Diesel engines the fuel-air mixture is ignited by high temperature of the compressed air. Let’s review the Diesel engine operating principle.
1-th stroke. The piston is moving down till lower dead point and the fresh air is blown from intake valve.
2-th stroke. The piston is moving up till upper dead point and the air in the cylinder is compressed multiple times (14 up to 25) and its temperature raises up to 700-800 C.
3-th stroke. At the moment the pistol reaches the upper dead point the fuel is injected into cylinder. Combustion act. The fuel-air mixture is expands and the piston is going down.
4-th stroke. The piston is going down, and gases are exhausted throw the open exhaust valve.
Diesel engine section: 1 – generator; 2 – air filter; 3 – fuel injector; 4 – fuel pipeline; 5 – glow plug; 6 – HPFP; 7 – cooling fluid (oil) filter.
The fuel in the cylinder ignites with rapid pressure jump that makes the engine to work noisy and with vibration. To preserve the operation safety on the required level the Diesel engine is designed much more durable than the petrol engine. More durability assumes more heavy-weighted.
The fuel supply system of Diesel engine also differs from the petrol-powered engine. The first thing to say it is more complicated. The fuel in the combustion chamber should be injected under high pressure and it is very minor in volume. The engine is controlled by the electronic unit, which controls the fuel pump and the injector according to data get from sensors. That kind of design makes the engine much more expensive.
The main advantage of diesel engine is the inexpensive fuel. Then the cost difference between diesel fuel and AI-98 petrol reaches the 20% level even the owners of expensive cars start to save. The fuel efficiency of diesel engines is 36% and higher. The efficiency of petrol-powered engine is only 25% that means that the diesel engine wastes less fuel than the petrol-powered engine.
The diesel engine as compared to the petrol-powered engine of the equal volume of combustion chamber has advantage of higher torque. It is critically important for minivans and station wagons. No alternative engines are mounted now on trucks. High torque helps in the city traffic; it allows moving on minimal engine turnover in traffic jams.
The basic disadvantage of diesel engine is that it is expensive. It expensive both in manufacturing (due to high work load) and also in maintenance. It is expensive due to ecological incompatibility of its exhaust and due to necessity to adjust its exhaust according to strict requirements of international agreements.
The fuel in diesel engine is ignited by the heat of the compressed air. It results in that fact that fuel had no time to fully mix with the air and it produces CH, NOX and carbon black during the combustion process. The carbon black is particularly visible then it colors the exhaust in black. And if in the case of hydrocarbons it can be removed by catalyst, the quantity of carbon black in the exhaust is adjusted by the special exhaust filter, which is mounted between the exhaust collector and the catalyst. The exhaust filter is warming up in the flow of exhaust gas that results in carbon black afterburning. Periodically the residual carbon black should be burned up and on the command block instruction the gas temperature is raised at the end of the combustion stroke due to burning of an additional quantity of fuel.
The catalyst have more complex design due to irregular chemistry of the exhaust gases.
Now let look at the problems concerning diesel fuel. As it’s known, the diesel fuel occurs to be of 2 kinds – summer fuel and winter fuel. They differ in the temperature of solidification. When the fuel freezes the fuel pump is unable to flush it and that’s the end... You are standing idle on the road shoulder unable to start the engine. This can be avoided by warming up the fuel piping (also fuel tank for trucks). Contrary to diesel fuel the petrol is non-freezing.
HPFP of diesel engine is extremely unreliable unit. Due to its operation on high pressures the ingress of water into the fuel is a mortal danger. Therefore water separator is required. The small particles of dirt also can damage the pump, therefore the filter after filler is necessary. In Russian environment 2 filters are required due to very dirty diesel fuel in Russia and not all resellers provide autos equipped with such engines to Russia. Such complexity of engine systems results in high prices on diesel engines – sometimes the price difference compared to petrol-powered engines is up to 4000 Euro.
Noise and vibration till the latest times could not be separated from the words “Diesel engine”. Attempts to neutralize them are in wrapping up the engine compartment by acoustic insulation, balancing the engine moments and calibrating the control units.
Why Diesel engines are still popular?
- The first, the ecological regulations are kept in foreign countries and the owners of ecology-friendly autos have discounts on assurance and other taxes.
- Secondly, on condition of quality oil fueling and maintenance on the regular base diesel engine can operate up to half-million kilometers without capital repair. And that is the sure gain.
- The third, the turbo-supercharging diesel engine can surely play the role of “fire-starter”. Many car manufacturers follow that way.
So if you think whether to buy diesel-powered car or not, you have to drive at least couple dozens of kilometers and you will appreciate whether you like it or not.