How Efficient are Engines: Compression Ratio & Thermal Efficiency

What Percent of the Thermal Energy Produced by Fossil Fuel Combustion Contributes to the Locomotion of a Vehicle?

There is a conundrum with respect to the diesel-versus-gasoline-engine comparison. It is common knowledge in Europe, Asia, Australia, South and Central America — just about everywhere in the world outside the United States — that diesel engines are considerably more fuel efficient than gasoline engines. 

A diesel engine travels much farther on a gallon of fuel than a comparably sized spark-fired combustion engine can travel on a gallon of gas. A diesel engine will travel between 25% and 30% farther on a gallon of fuel than a comparably sized spark-fired gasoline engine will gravel on a gallon. 

And, that gap is widening.

What is not as commonly known is that diesel engines are also considerably more environmentally friendly than gasoline engines. The reason most people are unaware that diesel pollutes less than gasoline engines is that 1) most statistics with respect to gasoline vs diesel emissions are on a per-unit-of-volume scale and 2) people — again — are not aware that diesel gives drivers an additional third more miles per gallon than gasoline. 

The math is simple. According to the Environmental Protection Agency, per gallon, diesel emits about 22 pounds of carbon dioxide. A gallon of high octane gasoline produces about 20 pounds. (Low octane gasoline emits more than that.) That means, per gallon, diesel emits roughly 10% more CO2  than gasoline. But, that still means diesel emits between 15% and 25% less carbon dioxide per mile. 

Diesel is simply a better fuel with respect to the environment and fuel economy. Equally important, diesel engines are better — far more efficient — than gasoline engines. 

So why do Americans drive gasoline vehicles?

Why Diesel Engines are More Fuel Efficient than Gasoline Engines

Conventional wisdom says the reason why diesel engines are much more fuel efficient than gasoline engines is that diesel is a better fuel than gasoline. And, that is true. Diesel is a better fuel than gasoline, propane, methane (natural gas) and just about every other fossil, “clean” and alternative fuel for that matter because diesel has a higher fuel density. The fuel density — a.k.a., “energy” density — of gasoline. “The calorific value of diesel fuel is roughly 45.5 MJ/kg (megajoules per kilogram), slightly lower than petrol which is 45.8 MJ/kg. However, diesel fuel is denser than petrol and contains about 15% more energy by volume (roughly 36.9 MJ/ liter compared to 33.7 MJ/ liter),” explains the European Automobile Manufacturers Association. 

However, the fuel density of diesel is not the only reason diesel engines have greater fuel efficiency than gasoline engines. 

Diesel has between 11% and 15% greater energy density than gasoline, a sum that — while it does play a large role in the fuel efficiency of diesel engines — does not explain the fact that diesel engines travel between 25% and 35% farther than gasoline engines of comparable size on the same volume of fuel. 

So, the question is, where does the additional 20% to 25% in mileage efficiency come from? Again, if diesel is only between 11% and 15% more energy dense than gasoline how come diesel engines get 9 miles to every 6 a gasoline gets?

The answer is simple. Not only is diesel a higher density fuel than gasoline, diesel engines are higher-quality engines than gasoline engines. The thermal efficiency of diesel engines far exceeds that of gasoline engines.   

Quality Comparison of Gasoline and Diesel Engines

There are a variety of means by which the “quality” of an engine can be defined. Torque and acceleration are two example of standards by which engine quality can be judged. But, with respect to combustion engine efficiency, there are only two relevant variables.  

With respect to combustion engine efficiency, quality is determined by 1) longevity — how long the engine will last — and 2) production — how much power the engine puts out in relation to the power potential of the fuel it takes in.

Engine Efficiency: Why Diesel Engines are Better than Spark-Fired Gasoline Powered Engines

Diesel engines — relative to other types of engines — are exceptional feats of engineering. From the advent of the first diesel engines up to those manufactured today, diesel has always outperformed gasoline engines with respect to engine efficiency. Diesel engines have always been higher quality. They have always lasted longer and diesel engines have always been more fuel efficient. Most importantly, diesel engines have always polluted less.

And, today’s engineers are separating diesel engines from gasoline engines even more.

Today’s diesel engine engineers are breaking down the barriers of thermal efficiency limitations. That is to say, diesel engines produce more kinetic energy from the heat produced during combustion than any other type of engine. 

With respect to combustion engine production, “The diesel engine has the highest thermal efficiency (engine efficiency) of any practical internal or external combustion engine due to its very high expansion ratio and inherent lean burn which enables heat dissipation by the excess air.” 

The reason diesel engines are more efficient at converting the heat produced during combustion into kinetic energy is that of how diesel combusts in a diesel engine

Compression vs Spark-Fired Combustion

There are two means of combusting a fossil fuel in an engine: with a spark or by compressing it to the point it ignites. All diesel engines are compression engines. All gasoline engines are spark-fired. That means compression is the catalyst of diesel combustion in an engine and a spark ignites the gasoline in a gasoline engine combustion chamber. 

The difference between the two means of combusting a fossil fuel in an engine is important because the two means generate different rates of combustion efficiency. 

Compression engines burn fuel more efficiently than spark-fired engines. 

Why Compression Engines Burn Fuel More Efficiently than Gasoline Spark-Fired Engines

Compression diesel engines and spark-fired gasoline engines have a great deal in common including injectors, pistons, piston cylinders, and exhaust outtakes. And, both compression and spark-fired engines are designed with what is called the Otto Cycle. The Otto Cycle is a four-stroke engine cycle — 1) intake stroke, 2) compression stroke, 3) power stroke 4) exhaust stroke — that has proven to be the most efficient automobile engine design. 

A cursory explanation of compression and spark-fired engines require the noting of only one difference with respect to the two engine types: compression engines use pressure to combust diesel while spark-fired engines ignite gasoline with an electric spark.

Prior to entering the combustion chamber of an engine, both diesel and gasoline are converted into a gas state. When a gas-state fuel is compressed, it heats. Heat generated by compression is a function of the Ideal Gas Law. “The volume (V) occupied by n moles of any gas has a pressure (P) at a temperature (T) in Kelvin. The relationship for these variables, P V = n R T, where R is known as the gas constant.” 

Since temperature and volume are constant, increasing the pressure on a gas — i.e., decreasing the volume of a gas — increases the temperature. 

In the piston cylinders, both spark-fired engines and compression engines compress their respective fuels. However, diesel is compressed to the point that it combusts. While gasoline is also compressed, before it is compressed to the point that it combusts, a spark from a spark plug ignites the gas-state petroleum.

Compression Ratio of Diesel vs Gasoline Engines

On a volume scale, compression engines produce greater thermal efficiency — energy output divided by engine input — than spark fired engines. A large portion of the thermal energy produced by the combustion of gasoline in a spark-fired engine is simply lost as heat, heat that is not converted into kinetic energy, but rather, lost out the exhaust

The reason more energy is lost by a gasoline engine that is lost by a diesel engine is that compression diesel engines have higher compression ratios than spark-fired gasoline engines. 

Thermal efficiency can be represented by a difference in temperature. The temperature of the air that goes into an engine during the intake stroke is different — considerably less — than the temperature of the air forced out of the engine during the exhaust stroke. Subtracting the temperature of the intake air from the temperature of the exhaust equals the thermal efficiency. 

The Georgia Tech Department of Physics and Astronomy explains:

“Since the compression and power strokes of this idealized cycle are adiabatic, the efficiency can be calculated from the constant pressure and constant volume processes. The input and output energies and the efficiency can be calculated from the temperatures and specific heats.”

Under ideal situations — 100% thermal efficiency — exhaust would be the same temperature as the intake air. That would indicate that all the heat produced during fuel combustion was used to force an engine piston down. In reality, an engine that uses 30% the heat — a.k.a., energy — to force a piston down is relatively efficient. 

“The efficiency by which they do so is measured in terms of “thermal efficiency”, and most gasoline combustion engines average around 20 percent thermal efficiency. Diesel is typically higher–approaching 40 percent in some cases,” explains 

Simply, a compression engine converts a larger sum of the energy produced during combustion into kinetic energy than a spark-fired engine does because diesel engines can achieve higher compression ratios.

Why Compression Ratios Increases Fuel Efficiency

The relationship between adiabatic compression and fuel efficiency has to do with thermal dynamics and physics. Aina T., Folayan C. O., and Pam G. Y. Department of Mechanical Engineering, Ahmadu Bello University, Zaria, Nigeria explain, 

“Increase in compression ratio induces greater turning effect on the cylinder crank [6]. That means that the engine is getting more push on the piston, and hence more torque is generated. The torque gain due to compression ratio increase can be given as the ratio of a new compression ratio( -./) to the old compression ratio.”

Simply, the more a gas-state fuel is compressed, the smaller the area in which it explodes. That means a greater amount of force is exerted on a piston heat as opposed to the cylinder walls. If two engines have the same amount of fuel in their respective cylinders and the force created by the combustion in one engine is greater on the piston head than that of another, that engine will have a greater thermal efficiency. 

Diesel engines — though not those used in cars, pickups, and trucks — can reach extremely high thermal efficiencies. “Low-speed diesel engines (as used in ships and other applications where overall engine weight is relatively unimportant) can have a thermal efficiency that exceeds 50%.”

If Diesel Engines are so Much More Efficient, Why Do We Drive Gasoline Powered Vehicles?

It’s anyone’s guess as to why we make the decisions we do as individuals, but it is probably safe to say most Americans drive gasoline engines because we are accustomed to them. In addition, gasoline engines were both quieter and had faster acceleration traditionally. Today’s diesel engines are both quiet and extremely responsive, but that has not changed the purchasing habit of Americans.

So, it is also probably safe to say that marketing has a great deal to do with why Americans purchase gasoline-powered vehicles. 

Whatever the case may be, buying gasoline engines is a mistake. It is costly and diesel engines are far more environmentally friendly. 


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