Diesel engines are more fuel efficient than gasoline engines. That is to say, diesel engines get better “gas” mileage than comparably sized gasoline powered engines. There are two reasons why. First, diesel is a better fossil fuel than gasoline with respect to energy density. And second, diesel engines are better engines with regard to thermal efficiency.
And while gasoline and diesel — as fuels — are not getting better, at least not with respect to energy density, both gasoline engines and diesel engines are becoming more thermally efficient. But, diesel enginesare becoming considerably more thermal efficient while gasoline engines are only making moderate strides.
Considering the strides gasoline engineers would have to make with respect to thermal efficiency in order to make gasoline engines comparable to diesel engines — even if diesel engine engineers were to make zero progress, — it is almost a certainty that gasoline engines will never be as fuel efficient as diesel engines.
Why Diesel is a Better Fuel than Gasoline
With respect to the amount of energy produced per unit of volume, diesel fuel is a better fuel than gasoline. Per gallon or liter of fuel, there is more energy in diesel than there is in gasoline. The amount of energy in a gallon of fossil fuel varies. Just like there is a difference between gasoline and diesel, there are differences between the diesel and gasoline’s extracted from different parts of the world. Some diesels have more energy per gallon than other diesel and the same is true for different types of gasoline.
But, generally, it is agreed that there is roughly 15% more energy in a gallon of diesel than there is in a gallon of gasoline. The measure of energy in a volume of fuel is called energy density. Energy density is directly related to the number of hydrocarbon molecules in a fuel.
Gasoline has an energy density of 33,867 megajoules per meter cubed. Diesel has an energy density of 37,184 mj/m3. Again, those are rough estimates. Fuels with higher sulfur counts have lower energy densities. High octane gasoline and high cetane diesel also have lower energy densities as well.
Nevertheless, it is fair to say that diesel fuel is anywhere from between 10% and 20% more energy dense than gasoline.
Why Diesel is More Energy Dense than Gasoline
The energy density of fossil fuels is a product of hydrocarbons. The greater the number of large molecular hydrocarbons in a fossil fuel, the heavier the fuel and the more energy rich. That is why, for example, methane — a.k.a., natural gas — has such an extremely low energy density. Natural gas is comprised primarily of very small energy molecules. So small are the molecules and molecule chains in methane, in fact, that methane is a gas-state fossil fuel at room temperature.
Gasoline, too, is also made up of smaller fuel molecules. While a liquid-state fuel, gasoline is relatively low in energy density, particularly when compared to diesel. Diesel, on the other hand, is extremely high in energy in relation to most other fossil fuels. Diesel is made of large, long-chain hydrocarbons. Because of the size and length of the hydrocarbons in diesel, diesel has a high energy density.
But, the energy density of diesel is not the only reason diesel engines are more fuel efficient than gasoline-powered engines. Not only is diesel a better fuel, diesel engines are better engines with respect to thermal efficiency.
Why Diesel Engines are better than Gasoline Powered Engines
Diesel engines are more fuel efficient than gasoline engines because diesel engines have a higher thermal efficiency than gasoline engines. Thermal efficiency is, in layman’s terms, how much of the energy in a fuel becomes mechanical energy. Thermal efficiency is a measure of how much of the total energy potential of a fuel an engine can convert into energy that moves a vehicle down the road or that a piece of machinery converts into work.
All heat engines, be they steam, gasoline, diesel, natural gas, etc. are surprisingly inefficient.
The Thermal Efficiency of Diesel vs Gasoline Powered Engines
Thermal efficiency is technically defined as the combination of two laws of physics: the first and second law of thermodynamics. The first law of thermodynamics states that the energy output of a system cannot exceed the energy put into a system. The second law of thermodynamics states that no system can be 100% efficient because heat will always be lost.
So, thermal efficiency can be defined with a formula. Thermal efficiency is the amount of useful energy put into a system divided by the total amount of energy put into a system. The word “useful” is important because the laws of thermodynamics state that there is always a waste.
Of all the energy put into a combustion engine, only a portion is converted into useful energy. The rest blows out the tailpipe or is lost as heat into the environment around the engine.
Diesel combustion engines have a far higher thermal efficiency than gasoline combustion engines.
Why Gasoline Powered Engines Will Never Get Better Mileage than Diesel Powered Engines
Currently, gasoline engines have an efficiency of between 20% and 35%. According to Toyota, a company that is attempting to produce a high thermal efficiency gasoline engine, the thermal efficiency of, “most gasoline combustion engines average around 20 percent thermal efficiency.” According to the same, “Diesels are typically higher–approaching 40 percent in some cases.”
However, the Swedish vehicle engineering company MDPI, the thermal efficiency of both gasoline and diesel engines is considerably higher, though hardly as high as a layperson might assume, “Current production spark-ignition engines are working with brake thermal efficiency (BTE) about 30–36%, compression-ignition engines have long been recognized as one of the most efficient power unit, the current BTE of diesel engines can achieve to 40–47%.”
Toyota claims that it is developing a gasoline engine with a considerably higher thermal efficiency than typical gasoline engines, one with a thermal efficiency of 38%. If the MDPI is correct, the thermal efficiency of Toyota’s new engine is still more than 20% less efficient than a typical diesel engine.
Why Diesel Engines Have a Higher Thermal Efficiency than Spark-Fired Gasoline Engines
There are two reasons that diesel engines have a higher thermal efficiency than gasoline engines: compression ratio and air-to-fuel mixture. Compression ratio is a measure of the difference between the total volumes of a combustion chamber when the piston is at the bottom of its stroke to the volume of the combustion chamber at the moment the fuel combusts.
Compression Ratio and Energy Output
An increase in compression ratio increases fuel efficiency because of the higher the compression ratio, the greater the amount of energy released from a given volume of fuel. “Theoretically, increasing the compression ratio of an engine can improve the thermal efficiency of the engine by producing more power output.” Diesel engines have a compression ratio of between 14:1 and 25:1. A gasoline engine has a compression ratio of between 8:1 and 12:1.
And the problem is, the compression ratio is not something gasoline engine engineers can increase, at least not without decreasing fuel efficiency. It is possible to make gasoline engines with higher compression engines. However, as gasoline is highly volatile and unstable in relation to diesel engines, higher compression ratios mean that the gasoline will combust prior to the spark igniting it in the fuel chamber. The pre-combustion of gasoline is commonly called “knocking.”
While it is possible to prevent pre-combustion ignition by putting additives in gasoline in order to increase the octane of the fuel, high octane fuel is less energy dense than low octane fuel. The reason being, the additives that prevent pre-combustion do so because they have a lower energy density and, therefore, do not ignite as easily as gasoline.
In other words, increasing the compression ratio of gasoline engines is a catch 22. The purpose of increasing compression ratio is to increase the energy output per unit of volume of fuel. But in order to increase the compression ratio of gasoline, it is necessary to reduce the amount of energy in it. So, increasing the octane of gasoline in order to increase the compression it can withstand without combusting — something is done in order to increase the total energy output — actually reduces the energy output.
High Oxygen to Fuel Ratio
The second reason that diesel engines have a higher thermal efficiency than gasoline engines is that of the oxygen to fuel ratio. The greater the amount of air in an air-to-fuel ratio, the cleaner the fuel burns and the more energy it produces. Diesel engines can run on an extremely lean mixture. However, in a gasoline engine, a lean mixture causes problems.
The ideal air-to-fuel mixture of combustion engines is called the stoichiometric air-to-fuel ratio. The stoichiometric air-to-fuel ratio of gasoline engines is typical, “14.7:1, which means 14.7 parts of air to one part of the fuel,” according to Honeywell Garret. The stoichiometric air-to-fuel ratio of diesel engines can vary, widely. While the stoichiometric air-to-fuel ratio is 14:6, “diesel engines are not running under stoichiometric conditions normally. Typical operating ranges of diesel engines spread between an air/fuel ratio of 18 and up to 70, depending on the operation point.”
While gasoline engines almost always run as close to the stoichiometric air-to-fuel ratio as possible, Dieselnet.com’s Hannu Jääskeläinen and Magdi K. Khair explain in their article, Combustion in Diesel Engines, “Diesel combustion is characterized by lean overall A/F ratio. The lowest average A/F ratio is often found at peak torque conditions. To avoid excessive smoke formation, A/F ratio at peak torque is usually maintained above 25:1, well above the stoichiometric (chemically correct) equivalence ratio of about 14.4:1. In turbocharged diesel engines, the A/F ratio at idle may exceed 160:1.
Therefore, excess air present in the cylinder after the fuel has combusted continues to mix with burning and already burned gases throughout the combustion and expansion processes.”
Because gasoline engines always run close to the stoichiometric air-to-fuel ratio, just slightly richer, and diesel engines can run as lean as 160 parts air to one part fuel, the efficiency at which diesel burns is considerably greater than the efficiency at which gasoline burns. The cleaner the burn, the more energy produced per gallon. So, not only does diesel have more energy per unit of measure, a diesel engine uses a greater percentage of that energy to produce mechanical power.
Why Diesel Engines are More Fuel Efficient than Gasoline Engines? ›
Diesel engines get better gas mileage than gasoline engines because of the density of diesel fuel and because of the compression ratios of diesel engines, compression ratio being a catalyst of thermal efficiency and thermal efficiency playing a major role in fuel economy.Is diesel fuel engine more fuel-efficient than gasoline fueled engine? ›
A diesel engine is about 20% more thermal efficient than a gas engine. This directly relates to a 20% increase in fuel economy.Are diesel engines more effective than gasoline engines? ›
Diesel engines are still more efficient than gas engines, but less so for those who are mostly engaged in city driving. Diesel cars also have more torque, which results in better fuel economy along with more impressive acceleration.How much more fuel-efficient is diesel than gasoline? ›
Diesel engines are more fuel-efficient and have more low-end torque than similar-sized gasoline engines, and diesel fuel contains roughly 10% to 15% more energy than gasoline. So, diesel vehicles can often go about 20% to 35% farther on a gallon of fuel than their gasoline counterparts.Why does a diesel powered vehicle have significantly better fuel economy than a gasoline powered vehicle of similar size? ›
Relative to gasoline engines, diesel engines have higher compression ratios, more rapid combustion, and lower throttling losses and operate leaner. As a result, diesel engines have a greater thermodynamic efficiency and hence a higher fuel economy (lower fuel consumption) than gasoline engines.How is diesel more fuel-efficient? ›
Better fuel economy
The reason is that diesel fuel contains more energy than the same volume of petrol. The difference can be quite large: It's not unusual for a diesel engine to have an official average mpg figure of about 70mpg, compared with about 50mpg for an equivalent petrol model.
Because diesel fuel has high energy density and because diesel fuel has high compression resistance, diesel engines are more fuel efficient than other engine types. And, because diesel fuel burns more efficiently — more completely — diesel engines pollute less.Why is diesel stronger than gasoline? ›
Because diesel engines are built to withstand higher compression, they tend to be more reliable and last longer than gasoline engines. Diesel fuel is thicker than gasoline, and as such, provides more power and mileage per gallon.What is the main difference between gasoline and diesel engines? ›
The only difference between the two is how the explosions occur. Gasoline engines use a perfect mix of fuel and air that are compressed by the pistons and ignited from the sparks created by spark plugs. Conversely, in diesel engines the air is compressed before the fuel is directly injected into the combustion chamber.Why do diesels last longer than gas engines? ›
Diesel fuel is a type of distillate fuel that is essentially produced from crude oil, which gives diesel engines slower cylinder wear than gasoline engines. This gives diesel fuel lubrication properties that extend the overall lifespan of the engine.
How efficient is the most efficient diesel engine? ›
What is the maximum efficiency an engine can achieve? The diesel engine has a theoretical system efficiency of between 55-60%.Why is diesel so much more powerful? ›
The reason that diesel engines produce more torque than gasoline engines is that diesel fuel has greater compression resistance than gasoline. Because of the stability of diesel fuel — its compression resistance, — mechanical engineers can develop compression engines for diesel fuel.Which is worse for the environment diesel or gasoline? ›
The simple answer is diesel emits slightly more pollutants per gallon than gasoline does. But, as is often the case, the simplest answer does not tell the whole story with respect to diesel emissions versus gasoline. The reality is, gasoline both emits more emissions and is a greater polluter than diesel.What are the pros and cons of diesel fuel? ›
- Pros: Lower fuel cost, higher resale value, longer-lasting engine.
- Cons: Diesel vehicles are more expensive, diesel mechanics are more expensive.
Is diesel better than gas? While diesel trucks tend to offer more torque, more towing power, higher gas mileage, and longer-lasting performance, gas trucks are generally lighter, faster, and better at handling heavy payloads. Ultimately, your choice will depend completely on your needs.Why don't we use diesel instead of gasoline? ›
That's because gasoline engines cannot easily combust diesel fuel. Since diesel fuel is very low in octane, depending on the amount of contamination, the engine may run rough or start knocking. At that point, your best bet is to call a roadside assistance service to get towed to a nearby auto shop.Does diesel waste faster than gas? ›
Diesels can achieve 25-30% better mileage than a gas engine due to the higher efficiency of diesel fuel, while the direct fuel injection during the combustion process wastes little fuel.What are benefits of diesel cars? ›
- Excellent gas mileage.
- Longer range. Better fuel economy means you don't have to hit the gas station as often. ...
- Diesels have no spark plugs or distributors, so they never need ignition tune-ups.
- Better durability. ...
- More torque.
As a consequence of a higher compression ratio (longer stroke and turbocharger), diesel engines produce more torque which means you get more acceleration off the line.Why is idling bad for a diesel engine? ›
Unnecessary idling wastes fuel, causes air pollution and increases engine wear. An idling diesel engine produces much higher emissions than it would while using the same amount of fuel under load. Extended idling causes a build-up of soot inside the engine and results in a puff of black smoke when the engine revs.
What are the disadvantage of diesel engine compared to a gasoline engine? ›
Higher fuel costs – The higher cost of diesel fuel has been a hurdle to diesel vehicles being widely accepted. Diesel fuel can be up to 50 cents more per gallon than regular unleaded fuel.Is diesel fuel better for the environment than gasoline? ›
Diesel vehicles burn less carbon dioxide (CO2) than gasoline cars to cover the same distance in highway conditions. However, they also emit potentially toxic particulate matter that has been shown to significantly impact public health.How efficient are gasoline and diesel engines? ›
Diesel engines generally achieve greater fuel efficiency than petrol (gasoline) engines. Passenger car diesel engines have energy efficiency of up to 41% but more typically 30%, and petrol engines of up to 37.3%, but more typically 20%. A common margin is 25% more miles per gallon for an efficient turbodiesel.What is the advantage and disadvantage of diesel fuel over gasoline? ›
Although diesel fuel used to be cheaper than gasoline, it now often costs the same amount or more. Although diesel fuel is considered to be more efficient because it converts heat into energy rather than sending the heat out the tailpipe as gas-powered vehicles do, it doesn't result in flashy high-speed performance.What are the pros and cons of diesel vs gas? ›
Is diesel better than gas? While diesel trucks tend to offer more torque, more towing power, higher gas mileage, and longer-lasting performance, gas trucks are generally lighter, faster, and better at handling heavy payloads. Ultimately, your choice will depend completely on your needs.Why does diesel fuel cost more than gasoline? ›
The transition to less polluting, lower-sulfur diesel fuels in the United States affected diesel fuel production and distribution costs. The federal excise tax for on-highway diesel fuel of 24.3 cents per gallon is 6 cents per gallon higher than the federal excise tax on gasoline.