Rudolf Diesel's idea was to build a diesel engine which could provide higher efficiency than the regular steam engine. Based on Sadi Carnot's theory on isothermal cycle, an engine can work at more than 90% efficiency. So based on Carnot's theory, Diesel initially developed an engine with comparatively smaller dimensions.
Diesel's Patent:
On 27th February 1892, Rudolf Diesel applied for a patent on "New Rational Thermal Engines" at the Imperial Patent Office in Berlin. On 23rd February 1893, he received a patent document DRP 67207 entitled "Operating Process and Type of Construction for Combustion Engines".
Technical Difficulties:
As we know, Compression ignition engines require high pressure to compress and burn the fuel. To achieve 250 bar of pressure was not technically feasible during those times. In 1893, Rudolf Diesel finally succeeded in reaching on an agreement to work with Maschinenfabrik Augsburg-Nürnberg (MAN). However, the agreement contained concessions in terms of an ideal engine. The maximum pressure was reduced to 90 bar, and then later to 30 bar. This lowering of pressure naturally led to combustibility problems. Diesel's initial plans to use coal dust as an alternative fuel was rejected.
In the spring of 1893, MAN built the first uncooled test engine with gasoline as the fuel, because it was thought that gasoline would auto ignite more easily. The working principle of auto-ignition i.e. the injection of gasoline into the highly compressed chamber during the compression stage was confirmed in this engine.
In the second test engine, fuel was not injected and was atomized directly into the combustion chamber with the aid of compressed air. The engine was provided with a water cooling system.
In the third test engine, a single stage air pump was used for compressed air injection. The test results for this engine confirmed a high level of efficiency at 26.2%.
Mixture Formation in the first diesel engines:
1. Compressed Air Injection:
The first diesel engine from 1897 worked on compressed air injection. In this method, fuel was introduced inside the combustion chamber with the aid of compressed air.
The fuel injector had a port(1) for feeding the compressed air and another port for the fuel(2). With the help of a compressor, air was compressed and made to flow into the valve. When the nozzle(3) was open, the compressed air flowing into the combustion chamber also swept the fuel in. This two-phase flow generated very fine droplets of the fuel and this helped in auto-ignition.
A cam was used to actuate the nozzle in synchronization with the crankshaft. Since, the air compressed was high in pressure, a low fuel pressure was sufficient to ensure the auto-ignition process.
This process however is not feasible for high load vehicles and for higher speeds. This is because of the lower penetration depth of the air/fuel mixture into the combustion chamber. The low pressure of the fuel led to limited spray dispersion and hence the amount of air supplied could not be increased to increase power. The vaporization time of relatively large droplets did not permit increase in engine speeds. Nevertheless, this system was used in trucks at that time.
2. Pre-combustion Chamber or Indirect Injection Engine:
The Benz diesel engine employed pre-combustion chamber process. This process ensure partial combustion of the air/fuel mixture in a pre-combustion chamber before delivering the mixture into the main combustion chamber of the engine. The pre-combustion chamber has a hemispherical head and it is roughly one-fifth the size of the main combustion chamber.
The fuel is compressed at approximately 230-250 bar pressure. To ensure partial combustion in the pre-combustion chamber, only a limited amount of air is supplied in it. The partially burnt fuel at high pressure enters the main combustion chamber, where it mixes with the air, auto-ignites and burns, releasing energy. This process is also known as indirect injection and was used predominantly in diesel engines until the invention of direct injection.
3. Direct Injection:
As the name suggests, this process employed direct injection of fuel at high pressure into the combustion chamber, where it mixed with air and burned to release energy. It was not until 1960s that direct injection was introduced in the commercial sector. Passenger cars started using direct injection engines until the late 1990s.
Use of the first diesel engines in vehicles:
Due to its enormous size, diesel engines were not suitable for mobile vehicles in the beginning. In 1923, the first diesel engines were used in five tonne trucks by Benz & Cie. It was an 8.8 litre engine with four cylinders and produced 45 to 50 bhp.
At the 1924 Berlin Motor Show, three truck manufacturers showcased their models running with different systems:
- The Daimler diesel engine with compressed air injection
- The Benz diesel engine with pre-combustion chamber
- The MAN diesel engine with direct injection
In1926, MAN introduced the most powerful truck of those times with 150 bhp for a payload of 10 tonnes.
In 1936, Mercedes introduced diesel engine in its passenger car model 260D. It was a four cylinder engine with 45 bhp.
In 1903, the first ship to be fitted with a diesel engine was launched. It produced 25 bhp.
In 1913, the first train to be driven by diesel engine was launched and it produced a whopping 1000 bhp.
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