Today in History – August 9, 1898 -Rudolf Diesel patents the diesel engine in the U.S. 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.