A History and Overview of the Trucking Industry Part 5
Diesel Engine Systems
The four basic systems of a diesel engine are fuel, lubrication, cooling and intake/exhaust. Manufacturers have designed and engineered each of these systems their own way, but every diesel engine operates on these four basic principles. Each system plays a key role in maintaining engine performance.
In order to create combustion, the fuel system stores, filters and dispenses fuel into the engine. The lubrication system serves to minimize friction between moving parts of the engine and cools internal engine components. The cooling system circulates coolant throughout the engine. The coolant will absorb heat and is then pumped through the radiator which will in turn dissipate the heat into the surrounding air. The air intake system filter will filter incoming air and sends it to the intake manifold or turbocharger (if it is a turbocharged engine), and then on to the engine cylinders. The exhaust system lets spent combustion gases leave the cylinders via the exhaust manifold, turbocharger or exhaust pipe. It is then transferred to the muffler and tail pipe.
Fuel System
Diesel fuel on trucks can be stored in either one to four fuel tanks ranging in size from 55 to 300 gallons. The diesel fuel moves from the tanks to the filter and fuel pump via fuel lines. The lines are constructed of rubber which is wrapped by a wire braid and then enveloped by a cloth covering. Fuel lines may vary in size from ¼ to ¾ of an inch inside diameter. Some lines are only one foot long while others are 15 feet long.
In older gasoline engines, fuel is mixed with air in the carburetor before the mixture enters the cylinders. There is a butterfly valve in the carburetor which restricts the flow of air and fuel, creating a vacuum in the intake manifold. The mixture will enter the cylinders via the intake valves. Newer gas engines have fuel injection systems instead of carburetors, which function similarly to diesel fuel injection systems.
In a diesel engine, air first enters through the intake manifold and then goes to the cylinder where it is compressed. Fuel is then injected into the cylinder where it meets the hot air at the top of the piston’s stroke. Figures 00-00 and 00-00 show diesel combustion process.
The force that pushes the pistons and gives power to the engine is developed by the rapid burning of the fuel-air mixture in the cylinder. This is true for both diesel and gasoline engines. The way the fuel is ignited in a diesel compared to a gasoline engine is quite different. In a gasoline engine with fuel injection, the burning begins when the spark plugs fire and the flame front moves from the spark plug tip to the cylinder walls. This is a controlled and smooth burning. However, in a diesel engine, the air is already in the cylinder and compressed to a high temperature and pressure before fuel is forced through the injector and into the cylinder. It is a very fine spray that burns on contact with the air. The cycle of fuel into the cylinders perpetuates the engine running. In order to turn off the diesel engine, shut down the fuel flow to the injectors and it will starve the engine, which will make it stop.
Up until 1994, Caterpillar had a fuel system where all of the fuel that was pumped to the fuel injectors went into the cylinders. During that time, Detroit and Cummins diesels had a system that allowed excess fuel to be diverted from around the injector body and back to the fuel tank. This not only helped to cool the injectors, but it also warmed the fuel in the fuel tank. The warming of the fuel tank was a great feature when cold weather set in, as a cold tank of diesel fuel was susceptible to jelling and could cause a stalled engine.
Present-day Caterpillar, Cummins and Detroit diesel engines all have an electronically-regulated fuel injection system. Diesel fuel is fed to the engine by the injection system. The injection system measures the proper quantity of fuel required for each cylinder and injects it at the proper time in the cylinder cycle. It atomizes the diesel fuel and properly mixes it with the air in the combustion chamber.
Even though an electronic fuel injection system is named such, it is actually composed of many non-electronic parts. The mechanical components are the fuel injectors, primary and secondary fuel filters, fuel pump, check valves and an orifice in the fuel return line to maintain correct pressure in the system. The picture shown in Figure 00-00 shows the system and how it operates as it draws fuel from the fuel tank through the fuel filters, into the fuel pump suction side, exits the pressure side of the fuel pump through the electronic distributor unit (EDU) cooler and flows into the internal drilled passages in the cylinder head and then on to the injectors. Some systems have one fuel filter on either side of the fuel pump instead of two before it.
The electronic unit injector, or EUI, is also known as the injector nozzle. It performs three functions:
1. In order to ensure proper atomization of the fuel when it is injected into the cylinder, it converts the low fuel pump pressure into high injection pressure.
2. It measures and injects the proper amount of diesel fuel.
3. It allows continuous flow of fuel for component cooling while the excess returns to the fuel tank.
The electronic control module (ECM) times and meters the fuel flow. The fuel will enter the EUI body through holes that are in the body casting. Upon entering, fuel passes through a passage into the control valve and plunger area. The plunger goes up and down in the internal bore of the EUI body. The EUI has a rocker arm that transmits motion to the injector follower, which moves the plunger.
When the piston is about 2/3 of the way up the cylinder on the compression stroke, the EUI injector cam lobe begins lifting, causing the rocker arm to push down on the follower and the plunger. Right before injection starts, the ECM sends an electrical pulse to the injector solenoid. The solenoid will pull the armature up, closing the poppet valve and trapping fuel under the plunger and passages that lead down to the needle valve.
As the plunger continues its downward stroke, the fuel pressure will increase. When the pressure is sufficient, it unseats the needle valve and the fuel is sprayed into the combustion chamber. After a certain amount of time, the ECM will shut off power to the injector solenoid, which will then open the poppet valve, drop the fuel pressure in the injector to the level of the fuel pump and then repeat the cycle.
Diesel fuel has the tendency to become thick and jell at low temperatures, unlike gasoline. To minimize this problem, diesel additives are available but will not help with fuel that is already jelled. There are other additives available to “un-jell” a tank of diesel fuel.
Lubrication System
Oil and Its Properties
While all lubricants for vehicles are petroleum-based, they are not all equal. Lubricants have developed into many different and specialized products which require an understanding of the properties of each and its intended use.
The most important property is viscosity. Viscosity is a measure of how freely the oil flows as a function of the temperature. It is measured by the time required for a standard amount of oil to run through a standard-sized hole at a standard temperature. Oils designated with a “W” are to be measured at a cooler temperature.
A multi-grade oil has been modified with additives so that their viscosity remains more stable as the temperature changes. This will allow the oil to pass the test for several grades. For example, 10W30 is 10 weight oil with additives that help maintain the oil viscosity level so that it performs equal to a 30 weight oil at hotter temperatures.
Other important properties that may be specified is the ability of the oil to cling to a surface and/or the ability to hold tiny impurities in suspension. As machines became more complicated and sophisticated, lubricants needed to do more and function better under increasingly harsher conditions. Oils are now made to the engines they service based on their specific properties and the environment in which they will be working.
Every lubricant container has an American Petroleum Institute (API) label that lists the grade and service of the oil. The service letter contains a two-letter symbol. The first letter indicates a gas or diesel engine application, “S” is for spark ignition and “C” is for compression ignition engines. The second letter indicates the level of specification of the oil. Typically, the higher the letter (A being lowest) the better the performance. It is always safe to use an oil that is rated higher than what is specified by the engine manufacturer.
A synthetic oil is a petroleum-based oil that has been modified for enhanced performance and longer life. They are specified in some applications and do an outstanding job, but they do cost more. The user has to determine as to whether they are worth the added cost.