Brief Diesel History
Rudolf Diesel (Germany) invented the diesel engine and put it on display at the World’s fair in the year 1911. It wasn’t until around 1927 that Robert Bosch (Germany) developed the mobile injection system to be used in automobiles. In 1931, a Cummins powered Duesenberg, finished 13th in the Indy 500, using half the fuel of their competitors. Diesel engines were banned from Indy shortly after. In 1977 VW produced the first “high speed” diesel powered car, the VW Rabbit.
- -diesel ignites with heat generated from compression alone and does not need a spark plug (although some older diesel engines did come with spark plugs)
- -provide more torque that similar displacement gasoline engines
- -make good power at low RPMs
- -diesel has more useful energy, 30%-35% while gasoline has about 18%
- -produces almost 0 HC and CO emissions
- -can run much leaner than gasoline engines (fuel economy)
Disadvantages: (most of these are no longer an issue for modern diesels)
- -diesels can be difficult to start in cold weather
- -requires a more heavy duty starter and battery and battery to overcome high compression
- -diesel engines are expensive to produce
- -N0x emissions from high compression must be dealt with
- -particulate emissions must be dealt with
Diesel is made from crude oil just like gasoline, kerosene and jet fuel but diesel will not burn in the same way as gasoline. If you throw a lit match into a puddle of gasoline it will ignite, but do the same thing with a puddle of diesel and the match will go out as if you threw it into a puddle of water. So how does it run an engine? For diesel to become volatile it must be close to atomization. My teacher was showing us a diesel injector tester at school one day and asked me for a lighter. When he shot the diesel through the injector and lit the lighter the diesel lit up like he was spraying brake cleaner or some other flammable aerosol spray. Diesel and (close to) atomized diesel are 2 very different beasts.
Cetane is an indication of a diesel fuels ignition delay. Ignition delay is the time between when the fuel was injected and when it starts to burn. Some North American diesel does not meet auto manufacturers cetane specifications. (check your owners manual) If this is the case on your own diesel, consider switching stations or running a cetane booster, you should notice slightly more power and better fuel efficiency. However, exceeding the manufactures cetane rating will not show any power or MPG gains. Where I live the cetane rating isn’t displayed at the pumps. You may have to do some digging to find out.
Paraffin wax is added to diesel fuel to help lubricate moving parts in the fuel system. This is also one of the reasons why diesel begins to solidify or “wax” at lower temperatures. In colder climates fuel stations will switch from summer diesel to winter diesel which has additives in it to prevent waxing. You could run into problems if you fill up in the summer and let the car/truck sit until winter and try to start it. Unlike gasoline, if you handle diesel that has paraffin added to it you should notice your hands become very soft.
Diesel Injection System Functions
- supply clean, water-free fuel to the pump/injectors
- time diesel fuel injection to the correct cylinder
- pressurise fuel to overcome cylinder compression and make fuel atomization (or as close to atomization as possible) possible. Up to, but not limited to 36,000 psi
- control injection quantity
- control length of injection
- distribute fuel in the combustion chamber
Gasoline is considered a spark ignition engine because the compressed air/fuel mixture is ignited by a spark created by a spark plug. Similarly, diesels are considered compression ignition because the air/fuel mixture is ignited by the heat from compression alone. Diesels usually have a 16:1 to 22:1 compression ratio. Diesel fuel injection must be timed so that the mixture does not ignite while the piston is still moving up on compression stroke. This can cause serious damage to the engine.
I mentioned that diesels really ignite from the heat from compression. On a cold morning start there may not be enough heat in the combustion chamber to ignite the fuel on the first few cranks, or in extreme cases not enough to start at all. Modern diesels have glow plugs which are basically a resistor that heat up the combustion chamber before cranking. They will stay on longer if the engine is colder. Even in the summer time the glow light will come on for a predetermined period of time. This does not mean the glow plugs actually came on. Manufacturers program this into their systems to make sure diesel drivers don’t forget to glow before starting through the summer.
No Throttle Body
Diesel engines don’t need a throttle body. Engine RPM is controlled by the amount of fuel injected, as well as minor changes in injection timing. (although it could be said that a turbocharger helps to control air flow) One problem that arises from not having a throttle body is that there is no vacuum source. Diesel engines need a vacuum pump (pictured) to make vacuum for the power brakes and other vacuum controlled systems. On the VW TDI engine, (and possibly other diesels) in the main air stream there is a piece that looks just like a throttle body, but that is not what it is. It is called an anti-shutter valve, or ASV for short. The ASV blocks off airflow to the engine when the engine is being turned off to help shut the engine down.
Diesel Combustion Chamber
Modern diesels use a bowl in the piston as the combustion chamber. This design allows combustion to take place inside the piston. Fuel is injected into the bowl when the system wants combustion to start near TDC. Notice the machined sections cut out for the valves, this is done because of how close the piston gets to the head at TDC.
Types of Diesel Injection Systems
Although 99% of diesel injection systems inject fuel into an area that sees high compression, (about 300-600psi) direct injection systems are any system that injects fuel directly into the combustion chamber. This could be into an area in the head but usually the head is flat and fuel is injected into an omega chamber in the piston.
Indirect Injection (IDI)
Indirect injection is any system that injects fuel into a pre-combustion chamber, turbulence chamber or anywhere other than the main combustion chamber. The combustion process starts in these pre-combustion chambers and works its way into the main combustion chamber and across the piston.
PLN* An inline pump has a camshaft inside that pushes plungers to create pressure and deliver fuel to external injectors.
PLN* A distributor pump has a single piston to create pressure. Fuel is directed to the fuel line of the cylinder about to enter powerstroke.
System where the main engine camshaft has lobes specifically for the unit injectors. The cam pushes down on the electronically controlled injectors to make the high fuel pressure necessary for diesel fuel injection.
Hydraulically-actuated Electronic Unit Injection system. This system is a variation of unit injection but it does not need a camshaft to operate. Instead a high pressure oil pump, or HPOP, separate from the main engine oil pump supplies high oil pressure to the injectors to pressurize the fuel before injection. These injectors are also electronically controlled.
Most common system on modern diesel powered vehicles. It basically works like a gasoline direct fuel injection system but with much higher pressures. A mechanical pump makes the high pressure necessary for diesel fuel injection and delivers fuel to a “common rail” which supplies fuel to electronic injectors. These electronic injectors can fire many times for one power stroke.
*PLN means Pump Line Injector, basically means any system where a mechanical pump is in charge of making pressure, delivering fuel to external injectors via fuel lines, timing fuel delivery and regulating injection quantity based on inputs. (mechanical or electronic)
Not much different than a gasoline tank except for the bigger filler nozzle.
These pumps in or near the tank may look like a gasoline fuel pump but they operate on much lower pressure. Their main job is to help supply fuel to the main pump.
Catches impurities in the fuel before they damage the pump or the injectors. General rule is to change it twice a year. It may also have a water separator.
Could be a distributor, inline or tandem pump. (though tandem does not make the high pressure on a unit injection setup) Distributor or inline pumps are in some way driven by the crankshaft because they need to be timed so they can deliver fuel to the injectors at just the right moment. These pumps can only fire one shot of fuel per power stroke.
Pump to Injector Lines
Yes these are different from brake lines. Most brake lines are made by rolling stainless steal, making a seam. (weak point) Diesel injection pressures are much higher than what most brake lines would ever see. Diesel injection lines are seamless and they all must be the same length to maintain EXACT timing on all cylinders.
Turbochargers In Diesels
Most modern diesels come with a turbocharger. These can be vacuum actuated, self regulating wastegate or VNT/variable geometry. There can also be more than one turbocharger. Turbochargers are what really helped diesels get away from that preconceived notion that diesels are all slow. Diesels can typically run much higher boost levels than a gasoline equivalent. Also, since diesels usually don’t have a throttle body, they do not need a blow-off or diverter valve. For more information on turbos refer to the turbocharger/supercharger section here.
Diesel Emission Systems
Diesel engines produce different emissions than a gasoline engine. Diesels produce low amounts of hydro-carbons (HC) and carbon monoxide (CO) when the engine is running properly. However, because of the high compression pressures, combustion heat and lean air/fuel ratios, NOx emissions are produced. SOx emissions are produced from the sulphur in diesel fuel, even with low sulphur diesel fuel. Particulate emissions or “soot” must also be dealt with.
EGR stands for Exhaust Gas Recirculation. Its purpose is to take some of the exhaust gasses and put them back in the intake manifold. The exhaust gasses that get recirculated has been through the combustion process already and has little to no oxygen left to burn. This lack of oxygen cools combustion and reduces NOx, one of a diesels main emission concerns. It sounds strange to say that hot exhaust gas cools combustion but the exhaust temperature is actually much cooler than the temperatures the cylinder sees during combustion, and the lack oxygen causes a much cooler burn. If you have ever worked on a gasoline EGR system, you most likely wouldn’t recognise a diesels EGR system. Diesel EGR pipes are much larger than a gasoline, and it may or may not have an EGR cooler, which has coolant hoses going to/from it.
Some diesel systems require urea injection to meet emission standards. DEF stands for diesel exhaust fluid, it is a mixture of 32.5% urea and 67.5% water. The solution is injected into the exhaust to reduce NOx emissions. Urea is highly concentrated in human and animal urine. DEF manufacturers appear to be very secretive as to where/what they get their urea from, but the standing rumour is horses or cows.
So how does it work? Basically, when the urea is injected into the exhaust, it is divided into ammonia (NH3) and carbon dioxide (CO2) by the heat of the exhaust. When the exhaust reaches the catalyst, the ammonia (NH3) and NOx are converted to nitrogen (N2) and water (H2O).
DPF stands for diesel particulate filter, it is a ceramic filter that sits in the exhaust to catch particulate or “soot” emissions before they enter the atmosphere. The soot needs to be burned before it clogs the exhaust causing a loss in engine performance. Most systems have active regeneration, which heats the DPF to burn off the particulates when the system demands it.
Special Notes When Working on a Diesel
-if you see diesel leaking on the high pressure side, DO NOT put your hand near it. At that pressure diesel can fly through your hand like butter. Even if you get a minor cut from the high pressure diesel, GO TO THE HOSPITAL! You may feel fine now but it doesn’t take much diesel in your blood stream to kill you.
-bleeding the high pressure fuel lines: If you have had the fuel system open for any reason you may have to get the air out of the system before the engine will run. Air can compress, and may not allow full pressure from the pump to reach the injectors. You really shouldn’t crack the fuel lines to bleed but some times there is no other way. If you have to crack open the lines, make sure you hands are out of the way while cranking over the engine.