Just why does a Diesel engine get better fuel economy than a gas engine? Well, they work differently and burn different fuel, for starters. A gasoline engine uses a spark ignition system and a Diesel engine uses the heat created by a very high compression ratio to ignite the fuel. There is actually work underway, by Mercedes Benz and others, to operate a gas engine using diesel-like compression ignition, for at least a portion of the time.
There are a few reasons for the increased of most diesel engines when compared to gas engines. One reason for the lower fuel consumption is that diesel fuel has a higher energy density than does gasoline. For example, standard diesel fuel has an energy density of approximately 139K BTU per gallon. On the other hand, gasoline has fractionally less energy, about 125,000 BTU per gallon. As a way of comparison, much touted ethanol is far below either of these two, at about 85K BTU / gallon. That means that a gallon of diesel there has a greater ability to provide propulsive energy, if the efficiency of combustion is equal.
There is another primary reason that diesel engines tend to get better fuel economy than a gasoline power plant. This reason is due to the fuel delivery system of diesel engines; direct injection. Direct injection is more conducive to ensuring all the fuel atomized and is burned. Obviously more complete combustion leads to greater efficiency and lower fuel consumption. The same reasons have led to direct fuel injection systems being adopted for gasoline engines from various auto makers, including Cadillac, Saturn, Audi, and Pontiac. Expect to see wider adoption of this technology on gasoline engines in the near future as a way to decrease emissions and save fuel.
Differences in Power Delivery Between Gasoline and Diesel Engines
A diesel engine tends to develop power lower in the RPM range than an a comparable gas engine. In addition, the diesel will usually develop more maximum torque but less horsepower than a gasoline engine. This means that depending upon the use, the actual fuel economy figures from both gas and diesel engines can be very different indeed. Diesels are typically excel at lower, more constant speed operation while pulling heaver vehicles, while gasoline engines are better suited to lighter vehicles, variable speeds and rapid acceleration.
That partially explains the widespread use of diesel power plants in trucks. They will deliver much higher fuel economy than a comparable gasoline engine when towing heavy loads, and more easily climb steep hills when doing so. There are also durability advantages to diesel engines, especially in heavy duty applications. The operational differences between diesel and gas engines also helps explain the really high highway fuel economy figures returned by passenger cars with small, turbo diesel engines, such as the TDI powered VW Jetta (41 EPA Hwy MPG).
That’s why a diesel engine get better fuel economy than a gas engine.
Until next time………
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