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Today in History – February 23, 1893 – Rudolf Diesel patents the diesel engine in Germany. Rudolf Diesel’s goal was to improve on the efficiency of the gasoline-engine that used the Otto cycle. His new concept for the engine was to compress the air in the cylinder adiabatically. Once this high level of compression causes the air to reach the ignition temperature, the fuel mixture is injected and the fuel automatically ignited. This allows the diesel engine to be smaller and lighter than the traditional gasoline engine and would not require the use of an additional fuel source or spark plug for the ignition.
As shown in the graphic above, the ideal air-standard cycle can be modeled as a reversible adiabatic compression, followed by a constant pressure combustion process, then an adiabatic expansion as a power stroke and an isovolumetric exhaust. At the end of the exhaust, new air is taken in and the cycle begins again.
Piston engine efficiency is largely a property of the compression ratio (CR). Gasoline engines would like a higher compression ratio, but are stopped at about CR=10 by engine KNOCK, where the fuel air mixture auto-ignites before a flame reaches it, causing an explosion in the cylinder that the driver can hear. The explosion is too much for the automobile piston, piston rings, heat transfer, and connecting rods. One way to avoid the explosion is to increase the octane number.
With the diesel, the explosion is avoided as the fuel is injected “slowly” so that the burn rate is managed by the rate of fuel injection. However, this “non-premixed” combustion produces an abundance of soot particles as well as nitric oxide that causes large cities to take on a brown ting in the afternoons. This happens where I live, for example, in the San Francisco Bay Area.
As a solution, I am working on a new combustion strategy called Homogenous Charge Compression Ignition (HCCI). In this strategy, lean operations help the HCCI engine approach the fuel efficiency of diesel, while maintaining clean, low-temperature combustion that doesn’t require elaborate exhaust treatment to meet air emissions standards.
See the Engineering Pathway’s educational resources on the diesel engine and automotive engineering and design or visit the Mechanical Engineering Education Community site.
2 responses so far ↓
1 Bob Dibble // Feb 23, 2008 at 2:20 pm
There is another interesting feature of the Diesel engine. The fuel is injected into the center of the cylinder. There is a blanket of air surrounding the fuel. As the fuel ignites and burns, some of the the air is consumed while the rest of the air is pushed outward and into cracks and crevices, especially the zone between the piston and the cylinder walls. Pushing air into these cracks is ok, because it is air. In the spark engine aka the Otto Cycle, the fuel and air are premixed upon entery into the cylinder. With the spark ignition, the expanding fireball advances into the unburned mixture and at the same time, pushes the gases, ahead of the fireball, into the cracks and crevices. However, unlike the Diesel engine, it is both air and fuel that is pushed into the crevice. This air and fuel, about 5% of the total, does not burn, causing a loss of efficiency, but also, the trapped fuel is later blown out in the exhaust stroke, leading to emissions of unburned hydrocarbons and carbon monoxide. Several attempts to combine spark ignition with blanket of air around the fuel mixture have met with limited success. An example of such an attempt is DISC “Direct Injection Stratefied Charge” (aka GDI gasoline direct injection )
Interesting, Stuttgart Germany has been home for many engine people. Otto, Diesel, Wankel are from that area. Diesel was born in France and move back to Germany some years later.
There is controversy as to Diesel’s death, which occured on the eve of World War 1 . He was on a boat going from Europe to England, he disappeared, did he jump off or was he pushed off?
2 Albert P. Pisano // Feb 24, 2008 at 8:44 pm
Folks, if you’d like to see some interesting research in MEMS-based micro- and milli- Wankel engines, then check out the class website for ME219. Lecture 39 is a review of MEMS Wankel engine research conducted in my group up until 2 or 3 years ago. Interesting that internal combustion can span such huge differences in scale! See: http://www.me.berkeley.edu/ME219/Lectures.htm
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