What Determines Engine Fuel Efficiency: “Gas” Mileage of a Vehicle
Fuel efficiency is the product of two variables. Fuel density is one of two factors that determine fuel economy and thermal efficiency is the other. That is to say, fuel density and thermal efficiency determine the “gas” mileage of diesel, gasoline, biofuel, ethanol, and propane-powered vehicles.
Not all fossil fuels have the same amount of energy per volume unit of measure — gallon or liter. Likewise, not all of the fuel that goes into the “gas” tank of a vehicle burns. A portion simply blows out the exhaust because of engine inefficiencies. Furthermore, not all of the fuel that does combust/ignite/burn becomes mechanical power. Even though an engine may combust a fuel, not all of the energy goes toward turning the wheels. That is to say, not all of the energy an engine produces pushes a vehicle down the road.
A large portion of the energy combustion engines produce becomes waste.
Fuel density and thermal efficiency determine fuel efficiency because the more energy in a fuel and the higher percentage of the energy that becomes mechanical power, the better a vehicle’s “gas” mileage.
Fuel Density as a Factor in Fuel Efficiency
The energy density of diesel and gasoline are roughly the same. Energy density is a measure of weight. Per pound of gasoline or diesel, the amount of energy in the two is comparable. But, liquid fossil fuels are sold in volume units — gallons and liters, — not by weight. Measured in volume rather than weight, diesel, and gasoline are different with respect to energy content. That is because diesel is denser than gasoline.
There is more energy in a gallon or liter of diesel than there is in a gallon or liter of gasoline.
The density of a fuel — as it relates to fuel efficiency — is a quantitative variable. The more energy in a fuel — per unit of volume, — the greater the fuel efficiency. And, the more energy there is in a fuel per gallon or liter, the better the “gas” mileage of the engine the fuel powers.
All fossil fuels have different fuel densities, considerably different densities in fact. Diesel, for example, has a fuel density that is between 15 and 20 percent greater than gasoline. According to Stanford’s Isaac Ramos, “When burned, these chemical compounds correspond to energy densities of approximately 155 million Joules per gallon for diesel and 132 million Joules per gallon for gasoline. Thus, in terms of energy density, diesel is clearly chemically ahead.”
Ethanol — one of the fuels thought to be a “clean” alternative to fossil fuels — has an energy density that is a third less than that of gasoline. The energy density of ethanol is almost half that of diesel. If energy density were the only factor that played a role in fuel efficiency and the price of all fossil fuels were equal, the cost of operating an ethanol-powered vehicle would be 1.5 times as expensive as operating a gasoline-powered vehicle.
However, ethanol is typically more expensive than both gasoline and diesel. It is not uncommon for ethanol to cost twice as much as gasoline and diesel for the same amount of energy. And, the price of ethanol can be expected to remain high because of the high production costs associated with it.
Not only is ethanol a fuel with less energy density than diesel and gasoline, it costs more.
The difference in the energy density of diesel vs gasoline is closer than the difference in energy density between gasoline and ethanol. But, even though diesel has an energy density that is 15 to 20 percent greater than gasoline, that does not explain why a diesel-powered vehicle gets between 25 and 35 percent better “gas” mileage than a gasoline-powered vehicle.
The remaining difference is the result of fuel density, a different measure than energy density.
Fuel Density: Combustion Efficiency, Compression Ratio, and Thermal Efficiency
In addition to the energy density of diesel fuel, diesel engines are more fuel efficient than other engines because the properties of diesel allow for it to combust in compression engines.
Gasoline, ethanol, and propane cannot power a compression engine. That means any vehicle or machine engine that operates on a fuel other than diesel is at a distinct disadvantage with respect to fuel efficiency because compression engines have a higher thermal efficiency than spark-ignition engines. The greater the thermal efficiency of an engine — the more energy that an engine converts into mechanical energy — the greater the engine’s fuel efficiency.
Compression engines have a higher thermal efficiency because they have higher compression ratios.
The compression ratio of an engine is the difference between the time when the piston is at the bottom of the cylinder and when it reaches the top of the cylinder immediately before combustion. The more the air in a cylinder compresses, the hotter it gets. The hotter the air-fuel mixture, the greater the thermal efficiency of an engine and, again, the better the fuel efficiency.
The compression ratio of an engine — how much the air or air-fuel mixture in a cylinder is compressed before combustion — is determined by fuel density. Compression is relevant because compression ratio determines thermal efficiency, the amount of energy put into an engine that becomes mechanical energy.
There is a limit to the ratio to which an engine can compress the air in a cylinder. The limits are not determined by how much pressure the piston in an engine can exert, but rather, by how much pressure a fuel can sustain before autoignition. Autoignition in a diesel engine is intentional. By design, diesel compression-ignition engines compress the air in a cylinder until it auto-ignites the diesel-fuel mixture. In spark-fired engines, on the other hand, auto ignition is a catastrophic event that will destroy an engine. In a spark-fired engine, at the very least, autoignition will cause severe damage.
Because autoignition in a diesel engine is intentional, not a malfunction, diesel engines can have much higher compression ratios. The compression ratios of diesel engines range from between 15 to 20:1. The compression ratio of a gasoline engine is limited to between 8 to 10:1. “Since the higher the compression ratio of an engine the higher the thermal efficiency, so diesel engines generally have a better fuel efficiency than petrol engines. To power a similar sized vehicle, a diesel engine will be about 15% – 25 % better on fuel efficiency than a petrol engine.”
How Thermal Efficiency Relates to Fuel Efficiency
Again, the thermal efficiency of a heat engine — a diesel compression-fired engine or a gasoline spark-fired engine, for example — is determined by the compression ratio. Thermal efficiency is a measure of how much of the energy goes into an engine becomes mechanical power and how much of the energy becomes waste.
GreenCarReports.com, “Most internal combustion engines are incredibly inefficient at turning fuel burned into usable energy. 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. Diesels are typically higher–approaching 40 percent in some cases.”
Heat is the reason compression ratio is relevant to compression ratio. The formula for thermal efficiency is the amount of energy — in the form of heat — that comes out of an engine divided by the amount of heat that goes into an engine.
When a piston compresses the air inside an engine cylinder during its upstroke, the compression generates heat. The more the piston compresses the air, the more heat generated. The closer the temperature of the air or air-fuel mixture to the temperature of the air-fuel mixture during combustion, the greater the thermal efficiency. If the temperature of the air-fuel mixture is the same prior to combustion as it is during combustion, the thermal efficiency is 100 percent.
But, fuel density and thermal efficiency are not the only factors that determine fuel efficiency. Combustion efficiency also plays a large role.
Gasoline engines do not combust even 90 percent of the fuel. And the amount of mechanical energy produced from the fuel that actually does combust is far less than 50 percent.
In order to burn, the hydrocarbons in fossil fuels — hydrocarbons being the valuable component of fossil fuels that ignite/combust/burn — require oxygenation. Oxygen and heat are the two variables that must be present for fossil fuels to burn. The ideal ratio of air-to-oxygen is called the stoichiometric ratio and it is 14 parts air to one part fuel in spark-ignition gasoline engines.
However, gasoline engines cannot operate using the stoichiometric ratio. The heat generated by the complete combustion of gasoline will cause a gasoline engine to overheat and seize. In other words, it is not possible to operate a gasoline engine completely efficiently. Instead, more fuel is added to the air-fuel mixture in order to keep engine temperatures low. Most spark-ignition gasoline engines operate using an air-fuel mixture of around 12:1.
Feeding an engine an air-fuel mixture that is slightly starved of oxygen is — in layman’s terms — called running “rich.” All gasoline engines are calibrated to run rich.
Saber Fallah of the University of Surrey explains, “the amount of emissions produced under lean mixtures is less than the amount produced by rich mixtures in a gasoline engine. Both gasoline and diesel fuels are a composition of hydrocarbons (made from hydrogen, oxygen, and carbon). They react with the oxygen available in the air to kindle burning. In an internal combustion engine, the air (oxygen) available in the cylinder can burn a portion of the fuel. If the amount of fuel is greater than the air available (rich fuel mixture), some unburned fuel will remain after combustion occurs. The vehicle expels the unburned fuel into the environment through the exhaust valves and tailpipe, consequently polluting the air and environment.”
The problem with running rich is that it is both inefficient and highly polluting. By adding more fuel to a mixture than the air can oxygenate, that means a portion of the gasoline blows out the exhaust unburned. The amount of gasoline from a full tank of gasoline that goes unburned and merely blows out the exhaust is roughly 14.3 percent.
In other words, because of the limitations of gasoline engines, 14 percent of the energy added to a gasoline engine is intentionally wasted.
Diesel engines, on the other hand, can run not only operate at the stoichiometric ratio, compression-ignition diesel engines can actually operate on a lean mixture.
Difference between Spark-Fired and Compression-Ignition Engines
In a spark-fired engine, gasoline is mixed with air prior to being injected into the cylinders of the engines. The mixture is injected into the cylinder when the piston is at the beginning of its cycle, bottom dead center. The piston rises and compresses the air-gasoline mixture and at the end of the piston cycle — when the piston is at top dead center — a spark fires and combusts the mixture.
In a compression engine, the air and diesel are mixed in the cylinder. As the piston begins to rise from the bottom dead center, it compresses the air inside the cylinder. When the air compresses, heat is generated. When the piston reaches the end of its cycle — top dead center — and the heat generated by the compression of air is at its peak, diesel is injected into the cylinder. The heat generated from the compression of air ignites the diesel and the energy generated from the combustion of the diesel drives the piston back down to the bottom.
So, while the air-to-fuel mixture in a spark-fired gasoline engine is always constant, the air-to-fuel mixture in a compression-ignition diesel engine varies. There is always the same amount of air in a diesel cylinder, but the amount of fuel added changes depending on how much fuel a driver injects by pressing and depressing the gas pedal. That means diesel engines run rich — during acceleration, — they run at the stoichiometric ratio — when idling —, and they run lean when a driver takes his foot off the pedal and the engine begins to brake.
Fuel efficiency — “gas” mileage — is the product of three independent measures of energy and engine inefficiencies. Simply, “gas” mileage depends on how much of the total fuel that goes into a car actually combusts; how much of the energy that combusts becomes mechanical energy; and how much energy a fuel has in it to begin with: combustion efficiency, thermal efficiency; and fuel density.