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Teknikmobil.com – This time we will try to explain several important things that car owners with diesel engines must know. Even though it looks the same as a petrol engine, in reality the diesel engine has a quite different concept. Therefore, we consider understanding these kinds of basics to be very important. We will start from the very beginning such as history, basics, construction, and some other important things. We hope that owners of four-wheeled vehicles with diesel engines will have basic knowledge after reading the following article.
Basics
In 1892, a German engineer named Rudolf Diesel perfected the compression ignition engine that bears his name. Diesel engines use the heat created by compression to ignite the fuel, so they do not require a spark ignition system or what we know as spark plugs (including coils and other circuits).
Internal Combustion Engine and Hybrid Vehicles
Spark Ignition and Car Engine Compression Ignition
Combustion occurs when fuel is injected into highly compressed hot air in a cylinder.
When the fuel burns, it expands and produces power. Due to the very high compression and torque output of diesel engines, they are made heavier and more powerful than petrol-powered engines of the same size.
The diesel engine uses a fuel system with a precision projection pump and individual fuel injectors. The pump delivers fuel to the injectors at high pressure and at timed intervals. Each injector sprays fuel into the combustion chamber at the precise moment necessary for efficient combustion. See image below.
One type of diesel fuel injection pump system
Advantages and Disadvantages
Diesel engines have several advantages over similar gasoline-powered engines, including:
1. More torque output
2. Greater fuel economy
3. Long live
Diesel engines have several disadvantages compared to similar sized gasoline-powered engines, including:
· Engine noise, especially when cold and/or at idle speed
· Unpleasant exhaust or smoke odor
· Cold weather startability or difficulty starting when the weather is cold
· Vacuum pumps are required to supply the vacuum requirements of heat, ventilation, and air conditioning systems
· Heavier than petrol engines
· Fuel availability
· Additional costs compared to petrol engines
This diesel engine weighs more than a petrol engine due to construction problems to withstand higher pressure .
CONSTRUCTION
Diesel engines must be built heavier than gasoline engines because of the tremendous pressure created in the cylinders during operation. The following table is a comparison between the two. The torque output of a diesel engine is often twice or greater than that of a gasoline-powered engine of the same size.
Comparison table between diesel engines and gasoline engines
AIR FUEL RATIO
In a diesel engine, the air is not controlled by the throttle like in a petrol engine. Instead, the amount of fuel injected is varied to control power and speed. Diesel air-fuel mixtures can vary from lean mixtures such as 85:1 at idle to as rich as 20:1 at full load. This higher air-fuel ratio and increased compression pressure make diesel fuel more efficient than gasoline engines, in part because diesel engines do not suffer as large losses. Throttling reduction involves the force required in a gasoline engine to draw air past a closed or partially closed throttle.
Meanwhile, in petrol engines, speed and power are controlled by the throttle valve, which controls the amount of air entering the engine. Adding more fuel to a gasoline engine’s cylinders without adding more air (oxygen) will not increase engine speed or power. In a diesel engine, speed and power are not controlled by the amount of air entering the cylinder because the engine’s air intake is always wide open. Therefore, the engine always has enough oxygen to burn the fuel in the cylinders and will increase speed (and power) when additional fuel is supplied.
NOTE: Many new diesel engines are equipped with a throttle valve. This valve is used by the emissions control system and is not designed to control engine speed.
One type of piston and connecting rod design in engines running on diesel/diesel fuel
DIRECT AND DIRECT INJECTION
In an indirect diesel engine (abbreviated as IDI/ indirect injection), the fuel is injected into a small prechamber, which is connected to the cylinder by a narrow hole. Initial burning occurred in this prechamber. This has the effect of slowing the firing rate, which tends to reduce noise. See image below.
Diesel engine with indirect injection
All indirect injection diesel engines require the use of glow plugs which are electric heaters that help start the combustion process. In a direct injection (abbreviated as DI) diesel engine, fuel is injected directly into the cylinders. The piston combines the pressure at which initial combustion occurs. Direct injection diesel engines are generally more efficient than indirect injection engines, but have a tendency to produce greater amounts of noise.
Diesel engine with direct injection
While some indirect injection diesel engines use glow plugs to aid cold starting and to reduce emissions, many direct injection diesel engines do not use glow plugs.
DIESEL FUEL RECOGNITION
Ignition occurs in a diesel engine by injecting fuel into the air, which has been heated by compression to a temperature greater than the ignition point of the fuel or about 1,000 degrees F (538 degrees C). The chemical reaction of burning fuel creates heat, which causes the gas to expand, forcing the piston to turn the crankshaft. A four-stroke diesel engine requires two revolutions of the crankshaft to complete the cycle.
1. On the intake stroke, the piston passes TDC, the intake valve opens, and filtered air enters the cylinder, while the exhaust valve remains open to some degree to allow all exhaust gases to escape the combustion process.
2. On the compression stroke, after the piston passes BDC, the intake valve(s) closes and the piston travels until TDC (completion of first crankshaft rotation).
3. In the power stroke, the piston approaches TDC on the compression stroke and diesel is injected into the cylinder by the injector. Ignition of the fuel does not begin immediately but the heat of compression initiates the combustion phase in the cylinder. During this power stroke, the piston passes TDC and the expanding gases force the piston downward, rotating the crankshaft.
4. On the exhaust stroke, when the piston passes BDC, the exhaust valve opens and exhaust gas begins to flow out of the cylinder. This continues as the piston moves to TDC, pumping the expelled gas out of the cylinder. At TDC, the second crankshaft rotation is complete.
Also read:
Construction and Types of Car Engines, What Are They?
Working Principles of Automotive Four-Stroke Engines
These steps will continue to repeat as long as the machine is running. If we look at a glance, these steps are not much different from a petrol engine. It’s just that the compression that occurs in a petrol engine is compression of fuel and air, whereas in a diesel engine the compression that occurs is only to compress the air. References and images: [Halderman,2012] Automotive Technology Principles, Diagnosis.[]