Diesels are on the rise, what makes them so special?
The fuel used in diesel engines is often referred to as fuel oil. This is because it is a longer hydrocarbon chain than those used to formulate gasoline. If some diesel fuel is spilled on the garage floor it will still be there the next day and will not have evaporated like a puddle of gasoline would have. So the fuel is what makes a diesel different from a regular engine. While it is true that the fuel is different, other differences obviously exist that necessitate a different fuel.
Diesels get better fuel economy than gasoline engines, and diesels usually go more miles than a gasoline engine before a rebuild is needed. Diesels make more noise then a gas engine, diesel powered cars are never as fast as gasoline powered cars (naturally aspirated), and diesels may have a tendency to produce nasty black smoke when they are not running right. Many of these things are generally known by anyone who has ever owned a diesel, and these things usually represent the extent of the average person’s knowledge regarding diesel engines but differences run much deeper, and there are reasons why all of the things mentioned above are true, or maybe not as true as they once were.
Various efficiencies affect the operation of every engine regardless of fuel type, and these same efficiencies actually have an impact on the operation many things mechanical. The first one is thermal efficiency. When fuel is consumed in an engine it is burned. This seems easy enough to understand but what is really going on is a conversion of energy. Engines don’t “make” power; they convert it from a potential source to a kinetic source. The energy is in the fuel, and when the fuel is burned this same energy is converted into something more useful which in this case is the motion that makes the vehicle go. More specifically though, the energy is extracted from the fuel by turning that energy into heat. More on how this efficiency affects diesel engine function in just a minute.
The fundamental difference between a diesel engine and a gasoline engine is the way that the fuel is ignited in the combustion chamber. A gasoline engine uses what is called spark ignition (SI) to get the fuel burning. SI uses a transformer style coil to produce a high-voltage spark and introduce that spark via a spark plug, into the combustion chamber. When the spark jumps the air gap at the tip of the spark plug, the heat of the spark ignites the air/fuel mixture. When the fuel burns, the rapid expansion of the hot gases pushes the piston downward in the cylinder.
In the case of a diesel engine the air and fuel are ignited by compression (CI). The compression itself doesn’t actually ignite the fuel, but rather the concentration of heat energy when all of the air in the cylinder is squeezed into a very small space in the combustion chamber. When the air is squeezed the molecules in the air start banging into each other creating friction. This increased molecular energy is manifested as heat. This heat is used to ignite the fuel which then expands and pushes on the piston.
In order for the temperatures in the combustion chamber to get high enough, the pressure in the combustion chamber must also be very high since temperature and pressure are always related. Because of this, diesel engines have a much higher compression ratio than gasoline engines. A gasoline engine might have a compression ratio in the neighborhood of 8:1 to 10:1 on average, but a diesel engine might have a compression ratio of 20:1. This higher compression ratio means that the engine must use more force to push the piston all of the way up but the result is combustion that produces a tremendous amount of torque, and it also causes more power to be extracted from the fuel then what you would get from the combustion process in a gasoline engine. This increased thermal efficiency is the reason that diesel engines get better fuel economy then similarly sized gasoline engines. Gasoline engines turn more of the energy from the fuel into wasted heat instead of movement of the piston.
|Cross section of a diesel engine. Note the large injector in the middle that |
sprays fuel directly into the combustion chamber.
With the compression ratio being so high in a diesel engine, the internal bits of the motor must be built to handle more abuse and greater forces. This tends to make diesel engines last longer than gasoline engines from a mechanical standpoint. Many of the heavy-duty diesel engines that are used in the big rigs that you see on the highways will go 500k miles or more before they need to be rebuilt, whereas the typical gasoline engine will only go half that distance before requiring a rebuild, and that’s only if you are lucky.
Because diesel engines have such high compression ratios they must also use a different method for putting the fuel into the combustion chamber. Gasoline engines will nearly always mix the fuel with the air, and then open the intake valve to let the two of them enter the combustion chamber mixed and ready to go. This means that the fuel is in the cylinder as the piston moves upward, compressing everything in its path and the fuel does not burn until the spark is introduced. Old gasoline engines used carburetors to dispense fuel into the top of the intake manifold. Multiport fuel injection involves spraying the fuel into the intake manifold right onto the backs of the intake valves.
In the case of the CI engine the fuel is injected directly into the combustion chamber once the piston has moved all the way to the top of the cylinder and all of the air in the cylinder has been thoroughly squeezed into a very small space. The reason that the fuel is injected in the manner is related to the high pressure in the cylinder. If the fuel was already in the cylinder as the piston was squeezing everything together it would spontaneously combust at a point before the piston was at the top of its travels, and ready to accept the force of the rapidly expanding gases. Because the fuel must be injected into this high-pressure situation, the injection pressure at the tip of the injector must be very high. On many newer diesel engines this pressure is around 25,000 psi. In order to inject the fuel at this pressure some very special fuel system components and strategies are required. The injection pressure in a gasoline engine is only somewhere between 30 and 60 psi.
Recently, new technology has been introduced in the world of gasoline engines that allows the SI engine to gain some of the efficiency of the CI engine. This new technology is referred to as gasoline direct injection (GDI) A GDI engine can run with a higher compression ratio but can still burn gasoline, and it can burn it in a manner that leads to more power on less fuel, with more of the fuel being properly consumed which in turn leads to lower emissions. This is a great development that will likely lead to the end of standard multi-port injection as we have known it for the last 30 years.
One of the draw backs to using diesel engines has been in the way that they respond to regular driving about town and on the highways. Diesel engines have traditionally produced an excellent amount of torque but not much horsepower. Think of torque as the force that does the work of moving the vehicle down the road, and the horsepower as the speed at which the engine can apply the torque. Because most people want better throttle response when they take off from a stop light, diesel engines have struggled to become as popular as gasoline engines.
Noise is also a consideration for many people. Diesel engines make a noise that can best be described as a loud deep rattle from within the engine. This noise is produced by the sudden combustion of the air/fuel mixture in the engine. When the air and fuel first ignite the initial blast is like a sudden explosion instead of a controlled burn. This little fireball does settle down and burn in a more controlled manner for producing reliable power, but when it first starts to burn it’s a bit erratic. The sound is a popping of the air from the sudden expansion and does not result from any mechanical parts banging against each other. On newer computer controlled diesel engines that noise is dramatically reduced by the injector squirting a tiny amount of fuel into the combustion chamber to get things burning before then spraying the main volume of fuel into the combustion chamber that is used to produce power. This tiny initial blast, referred to as a pilot injection, doesn't make much noise and it helps the large volume of fuel to begin burning in a much more controlled manner.
In reality the concern of snappy performance, or the lack thereof, that used to be associated with all diesels is really not all that valid anymore. Most diesel powered vehicles on the market now use a turbocharger to increase the horsepower of the engine and give it a much better throttle response. A turbo essentially puts more oxygen into the combustion chamber. The use of the turbo makes a diesel powered vehicle more pleasant to drive, but it does add extra cost to the bottom line when you go to purchase a new vehicle.
The question is often asked, why do we not have more diesel engine options if it works so well? In Europe at least half of all new cars sold are diesel powered, but in the US it is about 1%. The biggest reason that people in the U.S. don’t buy many diesels is because the cost of gasoline here is cheaper than what it is in Europe. The other reason is all of the terrible diesel engine options in the U.S. from the late 70’s and early 80’s. These vehicles resulted from the energy crisis of the 70’s but the problem is that many of these old diesel engines were horribly built.
|1984 Ford Escort. The reason that Americans don't like diesels.|
What could be worse than a 1984 Ford Escort? How about a 1984 Ford Escort with a non-turbo diesel engine. Many of the diesel engines from this time period didn’t use a turbo so they were gutless. They were not computer controlled so black smoke was normal without proper maintenance. And like all other diesel engines they were noisy. Some of them were just gasoline engines with some modifications to turn them into diesel engines. For the consumer this all meant higher initial cost to purchase the vehicle, less reliability because these engines were not well built, and hassle in finding a station where they could get fuel. Not all of these early diesel options in the U.S. market were that bad, Mercedes and Volkswagen had diesel engines in some of their offerings that were far superior to the ones in the domestic cars and trucks, but they were still gutless and noisy.
|Chevy Cruze. One of the new small sedans that will be|
available with a diesel engine.
With fuel economy again being a great concern, and with computer engine controls leading to greater efficiency and reliability, diesel engines are poised to make new inroads in gaining market share within the U.S. Companies such as Mercedes and Volkswagen have pretty much always had diesel options in some of their cars sold in the U.S. and the domestics have nearly always had a diesel option for their large pickups. Lately however many companies have begun to offer more diesel options in passenger cars where diesels had never been an option before. Chances are that diesels may yet do well in the U.S. but whether or not they can become as popular here as they are in the Europe remains to be seen.
- Better fuel economy, often much better
- High torque
- Built tougher
- No ignition system to wear out, i.e. spark plugs
- Higher purchase price
- A gallon of diesel costs more than a gallon of gasoline
- Not every gas station has diesel
- Usually slower than gasoline counterparts
- Good luck finding a good diesel mechanic that isn’t working on heavy-duty stuff only