Let me clear first, what Internal combustion engine does, it converts chemical energy into mechanical energy and heat is just an unwanted energy byproduct. The heat produced during combustion is just a “wasted” energy. Engine becomes more efficient when there is more conversion of heat produced by combustion to useful work.
How Diesel engine works
Diesel fuel comparative to equal volume of gasoline contain more chemical energy but this difference is not that much as I have ever thought. Hypothesis states that the difference is only 5%. But a diesel engine consumes 40% less fuel to produce same amount of power output as compare to gasoline engine with the same number of cylinders and total displacement. As we know more mechanical energy produced from same amount of fuel means less heat dispensed, since the chemical energy we have spent has to be accounted for thermodynamics equation balance.
A major reason that diesel engine is so much more efficient is the extremely high compression ratio. Means the diesel engine compress the air more because the diesel engine is relying on the heat of that air to instantly combust the fuel as it’s injected into it, so its raising the air above the self ignition temperature of the diesel fuel wherever in a gasoline engine its compression of air and fuel mixture which we don’t want to rise above it’s self ignition temperature because we want to use spark plug to ignite it rather than compression. There is a lot of more oxygen crammed into the cylinder at the top of the compression stroke, so that the fuel is able to burn more thoroughly. A diesel engine is able to accomplish this because the fuel is not injected until the piston reaches the top of the compression stroke. It is only compression air, not fuel. A gasoline engine compresses the fuel/air mix, so the compression ratio is limited by the fuel’s ability to avoid self detonation or “pre-ignition”. (That’s the “pinging” a gas motor makes when the octane is too low, or something else goes wrong). By the way, many manufacturers are currently working on Direct Injection designs for gasoline engines which will greatly increase their efficiency and improve their emission performance.
Compression Ratio: Well having high compression ratios there’s a direct correlation between the efficiency of an engine and how much torque it produces so those higher compression ratios translate into more torque for the diesel engine.
Speed of combustion: in gasoline engine a spark starts the ignition of that air and fuel mixture and then the flame travels outward from that spark plug until it finally burns all of that air and fuel, versus in a diesel engine the fuel nearly instantly combusts as it’s injected into the cylinder so complete combustions occurs much sooner.
Bore versus Stroke: The third thing that the diesel and gasoline engine efficiency differ by is bore versus stroke and the fact that diesel engines tend to have a longer stroke length so if you are looking at gasoline engine it’s typically going to have wider bore but the piston is not going to travel up and down quite as much versus looking at a diesel engine, which the piston will travel up and down quite a bit but the bore won’t be quite as wide as on the gasoline engine.
Torque: The torque is simply force multiplied by distance so in the case of diesel engine we have not only got a larger force but we are multiplying that larger force with larger distance, the larger distance around the crankshaft, and so ultimately that’s going to translate into more torque produced. Hence the fact, the higher compression ratio and its turbocharged are ultimately to be the biggest driving factor along with quicker combustion for diesel engines.
Effect of heat produced in Gasoline and Diesel engine
Both Gasoline and Diesel engines like to be “Hot” in order to work efficiently. But with Gasoline engine the problem is “How do I get rid of all the excess heat the engine produces?”. However, with a Diesel, the problem is just the opposite. “How do I conserve heat in the engine so that the fuel burns more completely?”. On a cold day there’s hardly any coolant circulating through the TDI, and the cooling fans will never ever turn on! The problem of conserving heat also partially explains why the fuel mileage drops so much in the winter. (“Winterized” fuel containing less energy are also a problem.)
Clattering sound – The “clattering” sound of a diesel has nothing to do with the amount of heat produced. A gasoline engine is burning fuel vapor mixed with air. A “flame front” moves outward from the spark plug. It is more of a rapid, smooth burn than an explosion. A diesel engine, on the other hand, burns very finely atomized droplets of liquid fuel. Ideally it would begin to burn instantaneously as it enters the combustion chamber from the nozzle of the injector, but in practice there is always a little bit of “delay” and a tendency for the fuel to explode all at once. Hence the “clatter” you hear. Many diesels, including TDIs, use Pilot Injection to help or control the combustion noise. A tiny, metered quantity of fuels is injected at several degrees before top dead center in order to start an “early” burn which helps ignite the main injection more rapidly. That also help lower emissions. More complete burning means more power from less fuel, accompanied by lower emissions.
Why Diesel engine is considered more than Gasoline
Most of the Diesel engine powered vehicle get 20-40% better gas mileage compare to Gasoline engine vehicles. Its all because diesel is more efficient, that is per pound of fuel burned the diesel extract more energy. Even though the diesel engine has a higher peak temperature during the ignition and power stroke, the expansion ratio of the hot gas is significantly higher compared to those of gasoline engine, followed by a turbo expander and the resultant exhaust is cooler. Because of this higher expansion ratio of diesel engine as compared to gasoline engine, the working temperature of diesel engine is high and its exhaust gas temperature will be lower than that of gasoline engine.
Hence we can conclude that as in a gas car the gas burns slowly and because of this longer burn time it transfers more heat while in diesel engine fuel burns extremely quickly which transfers less heat.