Why is Diesel More Efficient Than Gasoline?

When you hear the word “diesel”, you likely think about big-rig trucks and heavy-duty construction equipment spewing thick clouds of black smoke into the atmosphere. However, diesel has grown to become a viable mainstream alternative to gasoline in recent years by cleaning up its act and cutting down on the amount of harmful emissions released as a by-product of burning the fuel. It also does not hurt that diesel engines are safer, simpler, and stronger than their gasoline counterparts. For a variety of reasons, diesel is more efficient than gasoline.


What is Diesel?


As one of the more versatile forms of fuel, diesel can be used by casual drivers in their residential vehicles just as easily as professionals powering heavy trucks, equipment, trains, ships, and aircraft. Without getting into a science lesson, diesel fuel is derived from a variety of sources – commonly petroleum, although other material such as biomass, natural gas, biogas, and animal fat can be converted into synthetic diesel. Because diesel is heavier on the molecular level, its energy density is greater than that of gasoline and most other fossil fuels, meaning that diesel in comparison contains more energy per unit of measurement.


Energy Density Explained


Since energy density is a primary factor in the energy efficiency of a fossil fuel, it is important to understand what it is and how it relates to diesel. Basically, energy density is the measurement of the amount of energy present in a volume of fuel. The density of energy in a fuel is dependent upon the size and number of molecular hydrocarbons within the fuel – the bigger, the better. If a fossil fuel is comprised of smaller energy molecules, its energy density will be lower as well. One of the most extreme examples is natural gas, or methane, which exists in a gaseous state at room temperature because its molecules and molecule chains are extraordinarily small.


Diesel vs. Gasoline


Although not as extreme as methane, the energy density of gasoline in its liquid state is comparatively low as a result of its smaller molecules, clocking in at under 34,000 megajoules per metre cubed. In contrast, diesel consists of a generous number of large hydrocarbons linked together through lengthy molecular chains, resulting in a strong, heavy fuel with a much higher energy density of just over 37,000 megajoules per metre cubed. On average, the energy density of diesel is 15 percent higher than that of gasoline, with some estimates reaching as high as 20 percent. Simply put, diesel is a superior form of fuel in terms of energy efficiency because it produces more energy per litre or gallon than gasoline.


The Diesel Engine


Energy-rich fuel is not the only reason that diesel is more efficient than gasoline. Due to its design and the technological advancements that have been implemented over the last century, the diesel engine can draw an incredible amount of power from that energy-rich fuel and stands as a more efficient engine than its gasoline counterpart. Unlike gasoline engines, which require spark plugs, diesel engines achieve combustion by compressing the air in the cylinders to scorching levels, causing the temperature inside to increase. When the fuel is injected into the chamber, ignition occurs instantly and delivers maximum power to the engine without fear of improper combustion or pumping losses.


Examining Thermal Efficiency


The key to the impressive fuel efficiency of the diesel engine is in its high thermal efficiency, defined as the measurement of a fuel’s total potential energy that can be converted by an engine into mechanical energy used to power vehicles and equipment. Thermal efficiency is the nexus of the first and second laws of thermodynamics, which state that a system’s energy output is lower than the energy input because energy will always be lost in the form of heat. All combustion engines are startlingly inefficient because much of the energy fed into them in the form of fuel is eventually lost as heat into the atmosphere or through the exhaust pipe when it is burned. However, diesel engines are remarkably thermal efficient in comparison with other engines.


Waste Not, Want Not


Compared to gasoline engines, diesel engines have a much higher thermal efficiency, which translates into greater energy efficiency thanks to less wasted fuel. The reason for the diesel engine’s greater thermal efficiency is twofold – the higher compression and oxygen-to-fuel ratios produce more energy per volume of fuel and convert more of it into useful energy. The compression ratio represents the difference in the total volume of the combustion chamber between the bottom stroke of the piston and the point of combustion. As the air is compressed faster and harder, it becomes super-heated and allows more power to be extracted from the fuel when it combusts. Since gasoline is more volatile, gasoline engines will never be able to achieve the compression ratio of a diesel engine, ranging between 14:1 and 25:1 in comparison to 8:1 and 12:1 for gasoline.


Air to Fuel Ratio


In conjunction with the compression ratio, the high air-to-fuel ratio factors into the thermal efficiency of the diesel engine. When there is more oxygen than fuel present in the mixture, the fuel is able to burn cleaner and generate more energy. Since diesel engines can function on a lean mixture featuring more oxygen, the fuel enjoys a much cleaner burn resulting in more energy produced per volume of fuel. In comparison, gasoline engines suffer from performance issues when the ratio is higher because they typically operate along the stoichiometric air-to-fuel ratio, which is the ideal mixture of 14.7:1, whereas diesel engines range between 18 and 70 parts air to one-part fuel.


The Evolution of Diesel


While there was a time when diesel was restricted to highways, farms, and construction sites, it has evolved to the point that it is a serious alternative to gasoline in any setting. As a result of its thermal efficiency, diesel is a more fuel-efficient choice than gasoline, offering a reliable and effective combination of energy-rich fuel and an engine that converts more of that energy into mechanical power.



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