QUESTION: Why is the diesel engine more fuel efficient than a gasoline engine?
Just the other day a customer was having a conversation with one of the Geno’s Garage technical staff. The topic was the better fuel economy of the diesel engine over the gasoline engine, and the subtle mechanical differences in engine architecture.
The customer was correct in noting these differences:
However the customer “danced around” the easy to remember, easy to explain response.*(scroll down for response)
- Diesels are “heat engines”, they’re designed to be more efficient.
- Diesels use turbochargers.
- Diesels operate with a higher compression ratio.
- Diesels have more structure to support the turbochargers and the higher compression ratio.
- Diesels have a better (higher pressure) fuel injection system.
- Diesel fuel has more BTU content than gasoline.
- Diesel engines operate at lower RPM.
- Diesel engines are almost always “under square” (longer stroke) more torque.
Here is where we toss this question back to you: For a given number of cubic-inches (or liters of displacement), why is the diesel engine more fuel efficient than a gasoline engine? If you would like some supporting documentation for your response, we have a recommendation: The Dodge /Cummins Historical Perspective, pages 6-11.
TDR members, the Dodge/Cummins Historical Perspective book is another benefit of your TDR membership – access to the publication is free with your membership – along with numerous others.
Considering a TDR membership? Annual membership to the TDR costs $35 for four print magazines, the annual TDR wall calendar, TDR forum access, and the following downloadable publications:
However, if you simply want the Dodge/Cummins Historical Perspective book, non-members can purchase a copy for $9.95. Click HERE to purchase the downloadable Dodge/Cummins Historical Perspective book.
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* Here it is (in the simplest of explanations). The answer to why the diesel engine is more fuel efficient than a gasoline engine.
The Injection System
“It’s in the injection system.”
Rudolf Diesel designed the heat engine to use the injection of fuel at the last moment to ignite the compressed air. Understanding the heart of the diesel, the fuel pumps, is the key to answering the fuel efficiency question.
The Gasoline Engine
A gasoline engine is what engineers call “stochiometric.” Stochiometric describes the quantitative relationship between two or more substances, especially in processes involving physical or chemical change. With a gasoline engine there is a stochiometric equation of 14 parts of air to one part of fuel. Remember, always 14:1. (Okay, modern gasoline development has some engines at leaner 16:1 and gasoline direct injection is now widely being used.) Whether at idle or full throttle, the fuel and air are (typically) mixed outside the cylinders in a carburetor or injection manifold, and the mixture is introduced to the combustion chamber via the intake valve, 14:1, always.
The Diesel Engine
Fuel and air in the diesel design are not premixed outside the cylinder. Air is taken into the cylinder through the intake valve and compressed to make heat. Diesel fuel is injected near the top of the piston’s stroke in an amount or ratio corresponding to the load on the engine. At idle the air-to-fuel ratio can be as high as 85:1 or 100:1. At full load the diesel still boasts a miserly 25:1 or 30:1 ratio! It is in the injection system where we find the key to the diesel’s fuel mileage superiority.
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TDRComm Staff Member
TDRComm , Apr 22, 2024dieselwagon, brucejohnson, Diesel85 and 1 other person like this.
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