Restoration of the cylinder head of a diesel engine
The diesel engine is a compression ignition internal combustion engine. In 1893, this method was patented by Rudolph Diesel..
Over the years, a variety of methods have been used to start and run the engine, such as prechambers, air chambers, glow plugs, as well as a method in which the engine was started with gasoline and then switched to diesel fuel.
If you have ever tried to start the engine in cold weather, you know that this required an engine heater or auxiliary fluid to start. Those engines were mostly not equipped with computers, and fuel pressures ranged from 3,000 to 6,000 psi (psi). For today’s engines with computer systems for controlling fuel consumption and fuel pressure from 12,000 to 20,000 psi, cold starting is no longer a problem, and the need for those traditional aids has disappeared.
The compression ignition fuel principle used in a diesel engine is the process by which fuel is injected into the cylinder after the air in the cylinder is compressed, causing self-ignition of the fuel.
The compression ratio in a diesel engine is higher than in a gas engine. Most diesel engines have a compression ratio of 14: 1 to 25: 1.
Until the 1970s, diesel engines were mostly naturally aspirated. Then, to achieve greater power and efficiency, a turbocharger was installed on the engine.
Most modern diesel engines have a direct injection system, which contributes to an increase in fuel efficiency by 25% compared to gas engines, and also reduces emissions of pollutants. In Europe, the share of diesel cars is about 60% of all vehicles, in the United States, where diesel is not so popular, about 30%. However, among trucks this indicator is 94%!
Today, diesel engines use a high-pressure fuel system (HPCR – High-Pressure Common Rail) and an electronic fuel injection control system (EUI – Electronic Unit Injection). These diesel engines have turbocharging and intercooler or variable geometry turbochargers, charge air cooling radiators with oncoming air flow and exhaust gas recirculation systems.
The cylinder head is usually made of cast iron, the most common material used for this purpose..
Not so long ago there were times when it was enough to take it to a small workshop and clean the valves to repair the cylinder head. If you needed something more complicated, for example, replacing valve seats, grinding surfaces or cleaning nozzles, you could contact the nearest machine shop. Some of these engines did not even have specifications for the maximum and minimum valve installation depths! The diesel engines that we used then were predominantly of domestic production, such as Caterpillar, Cummins, Detroit, John Deere and International.
Much water has flown under the bridge since that time!
Due to the influx of manufacturers from around the world – Komatsu, Volvo, Yanmar, Sisu, Kubota, Shilbura and others – auto mechanics are required to have all the data and technical specifications that can be found to repair the engine. In addition, it always makes sense to contact the machine shop or the manufacturer for any additional information that may come in handy..
Sometimes nozzle nozzles are installed in the cylinder head of a factory-made cylinder. A special glue is used for them, and their surface cannot be polished, since they protrude above the surface of the head gasket. These nozzle nozzles and glue are not commercially available; they can only be purchased with the new cylinder head..
Restoration of the cylinder head of a diesel engine.
To begin with, as you know, the first thing to do after disassembling the engine is to clean the cylinder head; For this, the workshop must be equipped with a lift. For example, a Cat C-15 engine weighs over 400 pounds.!
If you are not going to change the camshaft bearings in an engine with an overhead camshaft, you should not use your washing cabin or hot-cleaning bath for this work. In addition, today in many diesel engines there are internal and reverse fuel supply channels – a special approach is needed to clean them; you may need to remove and replace the o-ring plugs.
For a successful inspection or cracks, the cylinder must be clean and dry. When inspecting a specific model, the mechanic must know its weaknesses. When processing the cylinder head, it is better to use darker powder, against which cracks are better visible. The dry method still seems to be the best and most effective for fast and accurate processing. Do not rush at this stage, concentrate on what you are doing – this will help to avoid unpleasant surprises.
Using a small mirror, you can inspect the holes and the area of the nozzle sleeve. If you really find cracks in the cylinder, this does not necessarily mean that the block head will have to be thrown away. There are welding workshops that in some cases can brew and restore your cast-iron cylinder head on a professional level, and it will cost you much less than buying a new head.
These welding shops take over the welding of cylinder heads, blocks, transmission crankcases and final drive crankcases – that is, almost everything that is made of cast iron. Remember that if there is a crack in the cylinder head, then it will probably be quite difficult for you to find a good replacement, as some models are prone to crack formation.
But there is a comforting moment: those welding workshops that are still operating today have already very extensive experience in the field of car repair, and the result of their work can be relied on.
Midwest Cylinder Head (Nevada, Iowa) uses a unique method of welding cracks in the cylinder head. For such work, an acetylene torch, cast iron rods and flux for cast iron welding are needed. After cleaning and flaw detection to detect surface cracks, the cylinder head is placed in a furnace and heated to 1300 ° F.
Then, the areas with cracks are cut out with the tip of an acetylene burner, into which oxygen is supplied to form a heating flame. The crack looks like a white line in the center of the influx of molten metal. The welder continues to remove the metal until the white line disappears, or until he reaches the water jacket. After that, you can start the welding process..
The welding process should take place in the lower position, since the basic material must be melted before using the cast iron filler rod. The welder must be careful not to make a mortise seam before the base material melts at the right temperature. After welding is complete, bring the cylinder head temperature to 2000 ° F, then cool the head to room temperature and turn off the oven.
Most diesel engine cylinder heads have nozzle bushings that lead to the block head. One of the purposes of the sleeve is to isolate the compression from the cylinder between the sleeve and the nozzle.
The sealing area inside the sleeve is usually machined at the factory, and the depth to which the nozzle tip will protrude into the cylinder depends on it. The outer surface of the sleeve also has two or three sealing portions. These sections isolate water on the upper and lower or on the lower and middle parts of the sleeve. In the heads of blocks with internal fuel supply channels, the upper o-ring also isolates the fuel from oil, and the middle o-ring prevents fuel from entering the cooling system.
Before attempting to replace these bushings, make sure that you have the necessary equipment for this. You will need special tools to remove and replace the tube, as well as to process the inner surface of the sleeve. The nozzle bushings are installed in the cylinder head to facilitate the passage of coolant through the cylinder head. Remember that whether you change the sleeve or not, you also need to check the fuel system and cooling system for leaks under pressure.
Nozzle bushings are made of various materials, according to the manufacturer’s specifications; they can be made of brass, copper, stainless steel or cast iron. Be prepared for variety, as bushings and o-rings are regularly improved.
Valves and valve seats
It is in valves and seats that the hardest, most active work takes place inside the cylinder head. BE CAREFULLY CAREFUL OF THEIR CHECK! Remember that with proper maintenance, modern diesel engines can function properly even after 1 million miles..
I advise you to replace all valves, seats and valve guides if the engine has been running for more than 5000 hours, or 500,000 miles. You can save by purchasing spare parts on the secondary market, which in their technical characteristics are much superior to spare parts of the original manufacturer.
Often manufacturers’ price lists do not include all valves, seats and valve springs. You will find that some manufacturers issue newsletters for specific models, such as the D8K Cat with the D342 engine, according to which all exhaust seats must be replaced when restoring the block head.
Do not forget that your goal is the proper operation of the cylinder head for the entire time for which it is designed. There are specifications for each of the parts mentioned; make sure that they are strictly followed. You need to check for compliance with such standards as the height of the valve seat, the width of the edge of the valve disc, the clearance between the guide sleeve and the valve stem.
How many times have we heard from our customers that they cannot believe how well the engine starts and sounds after adjusting the depth of the valves. Depth of valve seating, as well as the thickness of the cylinder head are the most important parameters of the valve mechanism, affecting the duration of its operation.
If it is necessary to replace the valve seats, it will be most convenient to use a TIG welding torch to make a groove around the seat without using the filler rod, allow it to cool, and the seat will come out by itself. This method will in no way affect the diameter of the blind hole, unlike other methods.
For most modern diesel engines, replaceable valve guides are used, but some engines have guides built into the cylinder head. In this case, the mechanic must have the necessary tools to process and install a thin-walled guide sleeve in the head of the unit. We recommend a cast iron replaceable rail with an external diameter of 1/2 ?. Grind the top and bottom of the sleeve and drill holes as needed to fit.
The pneumatic hammer is most suitable for replacing valve guides. Remove the bushing, clean the bore of the valve seat and set the bushing to the desired height. If necessary, drill so that the clearance between the guide sleeve and the valve stem meets the norm.
One of the most effective methods for machining cast-iron cylinder heads today are discs with cubic boron nitride (CBN).
If you have ever tried to treat the surface of a cylinder with a combustion chamber in this way (for example, cylinders in a 6.5-liter GM diesel), then you know that the nozzle atomizer must be stationary in the cylinder head. After each stroke of the cutting part of the tool, it is necessary to check whether the spray nozzle has shifted.
Before deciding whether to mill the cylinder head, you need to consider many factors. Firstly, you need to find out the minimum allowable thickness by checking the technical requirements. If the head is curved, you will either have to throw it away, or find a workshop where it can be straightened by a proven method like ours. Go for it.
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