A History and Overview of the Trucking Industry Part 4
Incorrect operation of the engine is usually because of ignorance or poor driving techniques on behalf of the driver. Some drivers develop techniques such as not allowing adequate engine warm-up and cool-down times. In time, these erroneous practices will result in excessive wear and tear on the diesel engine. When drivers are not implementing proper techniques, these expenses can add up quickly after you factor in downtime, replacement parts and repair labor.
Trucking companies are well aware of operating costs associated with fuel consumption. Before engines were electronic, fuel consumption was directly proportional to engine horsepower. Back then, to minimize fuel costs engines were carefully picked based on their projected use and horsepower requirements.
A modern-day diesel engine is equipped with electronic fuel injection. These systems have helped engineers put together higher horsepower engines that are much more efficient than previous methods. As a result of this, trucking companies want higher horsepower engines to help reduce driver fatigue and help improve driver retention.
Small and large engines both have their advantages, depending on what they are to be used for. Smaller engines, (11 liters and less) are best for local deliveries and/or short turn turnaround runs on non-mountainous freeways. Larger engines (12 liters or larger) are usually used for mountainous or over-the-road-type trips.
Regardless of what size engine is used, economy and profit are optimized when the equipment is properly maintained and has a well-trained driver behind the wheel.
Torque and Horsepower
The definition of torque is the turning or twisting force developed by the engine. Torque is put out from the engine via the clutch, transmission, axle, drive line, wheels and the ground. Rim pull is the torque transmitted to the wheels and gets the truck going.
Horsepower defines the rate at which torque is applied. For example, lifting 300 pounds at a rate of one inch per second requires little power compared to lifting 300 pounds at a rate of one yard per second. The term “horsepower” is old, coined to define the amount of power a horse could put out.
There are a variety of different horsepower measurements that can be taken. The actual horsepower created by the engine is measured at the end of the engine crankshaft and is called brake horsepower (bhp). Gross horsepower is the brake horsepower of the engine without its accessories. Net horsepower is the brake horsepower rating when the engine has all of its accessories such as a fuel pump, alternator, air compressor and fan muffler. The amount of power generated at the tires is called wheel horsepower. It will be less than the net horsepower because of frictional losses of the power being transmitted through the differential gears and the transmission.
Any physical quantity can be measured in a variety of ways, although the only true way of comparing the results between two tests is to make sure the tests were performed exactly the same way.
The Society of Automotive Engineers (SAE) has developed and published a multitude of standards that are used throughout the industry and government as a way to standardize the testing and measurement of many vehicular parameters, including horsepower and torque.
Diesel Engine Theory
Nikolaus Otto, in 1861, developed the first internal combustion gasoline engine. Later on, in 1892, Rudolph Diesel made the first diesel fuel engine with the added benefit that it didn’t require an electrical system to ignite the fuel.
Diesel engine fuel is ignited by the heat that is generated as air is compressed, unlike a gasoline engine which requires an electric arc from a spark plug to ignite it. Rudolph figured that fewer components would make a more reliable engine, and the transportation and military agree to this day. See Figure 00-00
Diesel engines also have an advantage in that more mechanical energy can be extracted from each pound of fuel. The compression ratios are also higher, which makes a diesel engine more efficient than a gasoline engine.
The combustion process in the gasoline engine is smooth. Fuel burns from the spark plug out into the cylinder, which gives a controlled push to the piston. On the contrary, diesel fuel is injected into very hot compressed air and burns much quicker, or explodes. These types of engines are built stronger. The noticeable clattering noises they produce are the explosions going off inside the cylinders.
Spark plugs in a diesel engine are unnecessary because as the piston in a diesel engine moves upward on its compression stroke, the pressure of the air in the cylinder increases, which generates heat. This increase in pressure raises the temperature of the fuel and causes it to ignite.
Some diesel engines have glow plugs. Glow plugs are not the same as spark plugs; they pre-heat the engine to insure combustion temperatures are reached at start-up on cold days. A lot of today’s engines have much higher compression ratios which allow them to start even without the use of glow plugs.
The rated size of an engine is determined by the amount through which the pistons move and in which the internal combustion takes place. This displacement of the pistons used to be measured in cubic inches, but since the transition to the metric system, it is now measured in liters. It is not that uncommon to see 14 liter diesel engines in tractors while most car engines are smaller than 5 liters.
Another way to define engines is by their compression ratio. It describes the volume in the cylinder when the piston is at the bottom of its travel and has its maximum volume, to the volume of the cylinder when the piston is at top dead center (TDC). TDC occurs when the piston is fully extended and the cylinder has its least volume. For example, a 15:1 compression ratio means that at the top of the piston stroke, the cylinder volume is condensed into 1/15th of its original space. A gasoline engine usually has compression ratios ranging from 8:1 to 10:1, whereas diesel engines vary from 12:1 to 22:1. Typically, the higher the compression ratio, the greater the engine’s efficiency.
Both diesel and gasoline engines produce oxides of nitrogen and create hydrocarbon compounds that, when combined with sunlight, create smog. However, the big black smoke cloud seen from the exhaust when an older diesel truck accelerates actually does not produce smog. Although it looks like it does, it is mostly composed of harmless carbon particles that quickly settle on the ground. The big black smoke cloud is usually an indication that the fuel injection system is set too rich. All of the modern diesel and gasoline engines have computer-controlled fuel management systems that very much increase the engine’s durability, stability and fuel economy. When a modern diesel engine is properly maintained, it has virtually smokeless exhaust.
White smoke in the exhaust of a diesel engine is often seen when the engine is cold just after it is started. This white smoke is normal and mostly consists of water vapor (steam) that goes away once the engine warms up. When blue or white smoke is seen coming from a diesel engine after it is warmed up it usually is an indication that the engine needs major maintenance.
Four Stroke Diesel Engine
A four stroke diesel engine is designed to take in air, compress it, mix it with diesel fuel, ignite it, and then dispose of the exhaust. See Figure 00-00.
- Intake stroke: the piston begins at the top of the cylinder (TDC). As the piston moves in a downward direction, the intake valve opens up and air flows into the cylinder.
- Compression stroke: Upon the returning upward stroke with both intake and exhaust valves closed, the air in the cylinder becomes compressed. The compression increases the temperature of the air to about 1000° F.
- Power stroke: Just past the top of the compression stroke and as the piston is going down, diesel fuel is injected with high pressure into the cylinder. The fuel vaporizes and ignites. The resulting combustion increases the pressure and temperature in the cylinder and forces the piston downward, which in turn rotates the crankshaft.
- Exhaust stroke. Once the power stroke is completed, the piston starts to move up again and the exhaust valves open. The rising piston forces the exhaust gases through the open valves. The gases leave the cylinder and travel through the turbocharger, exhaust pipe and muffler.
Two Stroke Diesel Engine
The two stroke diesel engine combines the four strokes into just two as seen in Figure 00-00.
- Power-exhaust stroke. The piston at TDC compresses the air, bringing its temperature to 1000° F. At that instant, fuel is injected into the cylinder, ignites and forces the piston down, which in turn rotates the crankshaft. When the piston is about half way down the cylinder wall, the exhaust valve will open and the pressure from the combustion forces the spent gas out through the exhaust valve. While the piston continues downward, a series of ports open, allowing fresh air into the chamber.
- Compression-intake stroke. As the piston returns upward from the bottom of the cylinder chamber, more fresh air enters the chamber. When the piston reaches the halfway point, the fresh air ports are in turn closed off by the piston. The rest of the distance to TDC is the compression portion of this stroke. When the piston reaches the top of the stroke, fuel is injected into the super-heated air, combustion occurs and the cycle begins all over again.