Diesel engines are the workhorses that power man’s largest machines, from trucks, ships, and trains to tractors and irrigation pumps. Diesels are internal combustion engines that share many features with gasoline engines, but they generate much more power and function more efficiently.
Diesel Engine Components
In a four-stroke internal combustion engine, either diesel or gasoline, the piston completes four separate strokes (the full distance traveled inside the cylinder) while rotating the crankshaft.
In a gasoline internal combustion engine, air and fuel are first injected into the cylinders. The piston compresses the air/fuel mixture, increasing the pressure and making it highly volatile. At peak pressure, the spark plug ignites, resulting in a controlled explosion in the cylinder. The rapid expansion generates the power that forces the piston down the cylinder, turning the crankshaft and eventually the gears and wheels of the vehicle.
A diesel engine functions in much the same way as a gasoline engine, with subtle differences.
Air is first injected into the cylinder during the intake stroke, but at a much higher volume than how it is injected into a gasoline engine cylinder. During the compression cycle, this larger volume is compressed to a higher pressure than the gasoline engine.
A gasoline engine air/fuel mixture is compressed to about one-tenth of its original volume, while the air in a diesel is compressed to a volume from one-fourteenth to one-twenty-fifth of its original volume. The higher pressure creates a minimum air temperature of 500°C.
When the air reaches its maximum compression and temperature, fuel is injected into the cylinder and ignites instantly. No spark plug is necessary. The explosion pushes the piston and produces the power that drives the machine. The final cycle is the exhaust stroke that forces the exhaust gas out of the cylinder. The four-cycle process repeats in each cylinder, thousands of time per minute, depending upon the quantity of fuel injected.
Diesel Engine Efficiency
A gasoline engine is typically about 30% efficient at converting fuel energy into mechanical energy, while diesel engines can reach an efficiency over 45%. Several factors contribute to the difference:
- The diesel engine design without a spark-plug ignition is simple, which allows it to compress fuel to higher pressures. The higher compression ratios mean the fuel and air mixture burns more completely and, therefore, it releases more energy.
- Low power fuel consumption also contributes to the diesel engine efficiency. A gasoline engine consumes more fuel and less air at low power, while a diesel engine needs less fuel at low power.
- Diesel fuel has a higher energy density than gasoline due to its molecular structure.
- A diesel engine runs with less friction because the fuel is a better lubricant than gasoline.
The result is that a diesel vehicle can travel further on the same amount of fuel than a gasoline-powered vehicle.
Basic diesel engines can run on almost any hydrocarbon fuel, including biodiesel made from waste vegetable oil. The inventor of the diesel engine, Rudolf Diesel, even ran some of his early engine designs on peanut oil. Modern-day diesel fuel is composed of a low-grade, less-refined petroleum product derived from heavy hydrocarbon molecules with more carbon and hydrogen atoms.
Why Buy a Diesel-Power Vehicle?
The higher efficiency of a diesel engine makes it attractive, but there are many reasons why buying a diesel engine is a smart choice today.
Noise and Vibration
Diesel-powered engines have long been considered foul-smelling and noisy, occasionally emitting a clacking sound while producing excessive vibration. Early diesel passenger vehicles gained a bad reputation, which slowed their acceptance in the American car marketplace.
Recent developments in diesel engine technology and a mandated change to ultra-low sulfur diesel fuel have made a significant improvement. Modern diesel-powered automobiles run much quieter and no longer emit clouds of black smoke during acceleration.
Although these vehicles may still be less smooth and quiet as gasoline-powered cars, the noise level is acceptable by most standards.
Pollution and Emissions
Another factor that affected the early acceptance of diesel cars in America was their emissions. Diesels became popular in Europe but gained a bad reputation in the U.S. where emission standards are much stricter.
The internal combustion diesel engine converts the fuel’s chemical energy into mechanical power. Under ideal combustion conditions, diesel is a mixture of hydrocarbons, producing only carbon dioxide (CO2), water vapor (H2O), and the unused portion of engine charge air. Volumetric concentrations of diesel exhaust gases are typically 12% CO2, 12% H2O, 17% O2, and the balance N2. These concentrations will vary based on engine load. Increased load produces more CO2 and H2O and less O2.
Their emissions can include some pollutants that can adversely affect the environment and human health. These are created by incomplete combustion of fuel, the chemical reaction from mixture components at high temperature and pressure, combustion of oil additives and lubricating oil, and the burning of non-hydrocarbon components such as fuel additives and sulfur compounds.
Other sources can contribute to pollutant emissions, but these are typical of all internal combustion engines and are not unique to the diesel. They are produced in tiny concentrations, but they can contain highly toxic material, such as metals and other composites generated by normal engine wear or compounds emitted from emission control catalysts.
Diesel exhaust gases typically contain less than a one percent concentration of pollutants. A much lower level is achieved with modern diesel engines that implement emission after-treatment devices, such as particulate filters and NOx reduction catalysts.
The use of cleaner fuel and improvements in engine performance over the past 30 to 40 years have made diesel engines more environmentally friendly. Researchers have developed computer-controlled injection devices that monitor fuel combustion, leading to improved energy efficiency and reduced emissions. Significant progress has also been made in the reduction of carbon monoxide and hydrocarbon emissions with the use of new CRT particulate filters and catalytic converters.
The environmental footprint from diesel engines is also impacted by the amount of fuel consumed and the refining required. Diesel engine vehicles have always produced better mileage than their gasoline engine counterparts and, therefore, consume less fuel. As an alternative to gasoline, the use of diesel fuel can also offset the energy costs required for refining crude oil into gasoline.
Improved Engines with More Power
Most trucks are equipped with a diesel engine for a good reason. The fuel provides more low-end torque than a gasoline-powered vehicle, which translates to more towing power and more power off the line.
Initially, a gas-powered truck may accelerate faster, but diesel trucks maintain their power better once in motion. Diesel trucks are better suited for carrying heavy loads, often accelerating faster when fully loaded. For towing heavy items, they are ideal.
Diesel Cars Hold Their Value
According to research firm ALG, diesel cars held their value better than comparable alternatives. After 36 months, compact diesels maintained 63% of their value, while gasoline cars held 53% and hybrids, 55%.
Marketing analysts contend that diesel value performance can be attributed to sheer numbers. There are fewer diesel cars on the market, so a used diesel commands a higher resale price. They are attractive to car buyers for their fuel economy, especially when fuel prices are so high.
Better Gas Mileage
Per the U.S. Energy Information Administration, the average prices for fuel in the U.S. for the first week of January 2018 were as follows:
- Regular Gasoline: $2.522/gal
- Diesel: $2.996/gal
While diesel is more expensive than regular gasoline (about $.27 per gallon), it contains 25% to 30% more energy. The fuel efficiency savings can more than offset the higher price. For example, new diesel BMW 328d offers superior mileage to the Smart gas-powered Fortwo coupe. For an SUV or truck, the difference in mileage between diesel and gas will usually be lower than a car.
The primary motive for purchasing a diesel car is an improved fuel efficiency. Although the price of diesel fuel is higher than regular gasoline, the difference is much less compared to premium gasoline and location. Most drivers who would normally fill up on premium gas are willing to pay the slightly higher price for diesel because their trips to the pump are less frequent. Even with the slightly higher cost of diesel, yearly fuel costs will be lower.
Another consideration when choosing a diesel or gasoline vehicle is the type of driving. Diesel engine performance is at its best racking up highway miles. The gas mileage is significantly better on the highway than it is in the city with traffic lights and frequent stops.
Diesel Fuel Is Available Everywhere
Diesel engines have powered large trucks for decades and, as a result, there is a vast network of refilling stations across the entire U.S. Also, more than 50% of all American gas stations countrywide offer diesel. Abundant access to fuel makes choosing a diesel car an easier decision. The alternative electric vehicle offers a charging infrastructure that is still in its infancy.
Rugged and Reliable
Diesel engines are more fuel-efficient than their gasoline engine counterparts partly because they operate at much higher pressures. To maintain these pressures, the engine must be structurally stronger than a gas-powered engine, meaning they are also more expensive. Buyers pay a premium price for the engine’s quality and durability, but there are several benefits.
The toughness and stamina of a diesel engine make it ideal for off-road vehicles. A diesel engine is more durable than a gasoline engine and, on average, has a lifetime three times longer. Depending upon the adherence to a recommended maintenance schedule, a typical gasoline engine needs rebuilding after about 125,000 miles.
A diesel engine, on the other hand, can reach 375,000 miles before a rebuild is required. The author of Auto Repair for Dummies, Deanna Sclar, claims that several Mercedes-Benz diesel-powered vehicles achieved more than 900,000 miles before an engine rebuild was required. The long-lasting engine gives the diesel a distinct advantage for trade-in and resale value. Many used diesel trucks have plenty of engine life left.
Diesel fuel contains its own lubricants and is naturally heavier than regular gasoline. These characteristics contribute to less wear and erosion, prolonging the life of the moving parts. In addition, a diesel engine uses fewer moving parts than a gasoline engine, making it easier to repair. Less costly reconditioned parts are often used instead of new parts for repairs.
In many ways, diesel-fueled engines require less maintenance. They do not have complex ignition systems with spark plugs and distributors that require tune-ups. The high thermal efficiency also gives them more strength. As an example, most diesel trucks can haul up to four tons without any negative impact on fuel efficiency.
While these features are attractive to the potential buyer, maintaining the engine with regularly scheduled oil changes, along with air, oil, and fuel filter changes is necessary.
Failure to maintain a diesel engine properly with routine care can have dire consequences. Repairs to a broken fuel injection system, for example, may require the services of a certified diesel mechanic.
The European car market has always been enamored with cars powered by diesel engines. According to the U.S. Department of Energy, 49.5% of all new passenger cars registered in Western Europe in 2016 ran on diesel.
If German car buyers, who value the high-speed travel available on the Autobahn, are willing to trade their fast gasoline cars for more efficient diesel cars, perhaps more Americans should consider the change as well.
Any decision to purchase a diesel vehicle must be based on the overall cost and the value of that investment over time. The initial purchase price of a diesel car or truck is higher.
Once purchased, however, the process of cost recovery begins. The initial investment can be recouped at the gas pump over the long haul. Even with slightly higher diesel fuel costs, the superior mileage means lower total fuel costs.
A well-maintained diesel engine vehicle will generally outlast a comparable gas-powered vehicle. For many drivers, the purchase of a diesel engine vehicle is a smart choice that will yield many years of faithful service.