The nuts and bolts of the common rail engine
Simple steps could save you thousands in repairs down the line
Published 27/04/2016 | 02:30
Have you noticed that most tractors and equipment in Irish yards are powered by something called a "common rail" engine?
While the phrase is often used by overeager salesmen or weary mechanics, very few people fully understand the operating principles of the common rail fuel system and are thereby leaving themselves open to costly repair bills.
The common rail fuel system that we know today was developed by Fiat in the early 90s. However, due to financial difficulties they sold the rights of the system to Bosch for further development. In hindsight the move would prove to be a grave error due to the huge popularity of the design in modern engines.
The basic operating principle of any diesel engine, be it the old fuel injected style, or the newer common rail, is quite simple. Diesel is stored in the tank. From here, a low pressure pump delivers it to a high pressure pump near the engine. It is then introduced into the individual cylinders, where the rising piston compresses the air within the cylinder to such pressures that sufficient heat is created to ignite the fuel.
This creates an explosion within the cylinder, driving the piston back down again. If a four cylinder engine is idling at 1,000rpm, 2,000 of these explosions take place in one minute.
As a basic analogy to understand the difference between direct fuel injected and common rail engines, imagine cows coming to a parlour for milking.
In this scenario, you have enough cows to keep you milking for the full day, let's say 500 cows. The fuel injected system delivers pressurised fuel to the cylinder only when it is required. This is similar to going to the field of 500 cows, and bringing in just 10 cows to fill one side of the parlour.
When these 10 cows are milked, you go back out and bring in 10 more. You continue to do this, but there are never cows waiting outside the parlour.
You only bring in as many as you need. Imagine the herd of cows at the field gate, waiting to come in for milking as the fuel, being sent to the parlour (the cylinder in our analogy), along the farm roadway (the injector lines) only when they are required.
The common rail system is easily understood if we imagine it to be the collecting yard at the back of the parlour. The full herd of cows are waiting in the collecting yard. As soon as you open the gate at the back of the parlour, cows come into the stalls. There is always enough cows to fill the parlour. All you need to do is open the gate at the rear of the parlour, and the 10 cows walk in to fill the stalls.
You have a constant supply of cows (fuel), waiting in the collecting yard (the common rail), ready to come into the parlour (the cylinder) when the gate (the injector) is opened.
The fundamental differences between an old fuel injected engine, and a modern common rail engine is the pressure and manner in which the fuel is delivered from the high pressure pump to the cylinder, and the sophistication of the electronic control systems governing the delivery of fuel to the cylinders.
The fuel injected engine features a pump which delivers fuel to the injectors through individual lines, at about 200 bar pressure.
Many people will be familiar with slackening these fuel lines off the injectors as a method of bleeding an engine, should it become air locked.
With this system, the pump only pressurises the one line feeding the cylinder requiring fuel at any given moment in time. The remaining fuel lines will not be fully pressurised until the individual cylinder it is feeding requires fuel.
The term common rail refers to a chamber where fuel is stored, at very high pressure, prior to entering the cylinders.
The high pressure pump delivers fuel to this rail, where it is stored at approximately 2,000 bar (29,000 PSI).
When any individual injector is opened by the ECU, some of the pressurised fuel exits the rail, and enters the cylinder via the open injector.
Meanwhile, the fuel pump is constantly working to replace the lost fuel and keep the pressure maintained in the common rail.
To learn more about the pros and cons of the system, I recently paid a visit to Hugh Collins at Atlantic Diesel Services near Tralee.
I began by asking him why there was a need for such massive operating pressures.
"It all boils down to burning fuel as cleanly and efficiently as possible," he says.
"Injecting fuel into the cylinder at a very high pressure allows it to be sprayed in as a very fine mist, leading to a hotter burn, more power and lower particulate emissions."
However, this high operating pressure brings issues of its own.
First and foremost is the safety aspect of the system. Let's be absolutely clear on this issue, an untrained professional should never tamper with an operational common rail fuel system.
Fuel escaping at 2,000 bar has the potential to sever bone. If you were to run your hand over a jet of fuel leaking from this system, it will simply sever your hand where it strikes you.
The safety issue aside, there is the problem of manufacturing components capable of generating and working with this pressure for thousands of hours.
"For a pump to generate 2,000 bar, and for an injector to hold that pressure back, all components need to be manufactured to extremely precise tolerances," explains Mr Collins.
To get the full value of the high pressure fuel, it must enter the cylinder through microscopic holes in the tips of the injectors.
The smaller the hole in the injector, the finer the mist of fuel as it is sprayed into the cylinder.
A fine mist is the name of the game.
For comparison, a human hair is about 30 microns in diameter, the holes in the injectors are approximately 60 microns.
On the day of my visit, Mr Collins was busy working on a set of injectors from an excavator which had clocked up just 4,000 hours.
The high pressure fuel pump, the common rail and all six injectors needed replacing, costing approximately €4,000 at a conservative estimate.
I asked him if there was any steps the owner could have taken to prevent these costly repairs.
"I can never stress the importance of a good filtration system enough to customers," he says. "We're dealing with a highly sophisticated fuel system, yet the fuel tank in the yard hasn't been updated within the last two decades."
While a stick under the outlet side of the tank to throw the dirt to the back might have done the job back in the eighties, it doesn't cut the cloth any more.
As a basic requirement, every bulk storage tank should have three filters.
There should be a breather filter to prevent condensation from drawing in through the lid at night, a two or three micron filter to trap dirt and crud on the outlet, and a water trap.
To set up these three filters on your tank, you're probably looking at €100.
A stitch in time saving nine comes to mind.
Aside from filtering the fuel through your fuel tank, you need to pay careful attention to filters you use on your machine. Mr Collins highly advises against the use of spurious filters.
"The price difference is very minor, but the difference in quality is massive.
As an example, the legitimate filter for a Ford Mondeo is a 2 micron filter, but the best spurious filter available for that car is a 12 micron filter".
There are plenty of places to make sensible savings around the farm, but don't try it with the integrity of your fuel system.
Atlantic Diesel Services are based in Tralee, Co Kerry and can be contacted on 066 7193200.
FUEL AND FILTER CHECKLIST
Ensure you buy your fuel from a reputable source. Poor quality fuel lacks in lubrication. Don’t forget that the fuel serves the additional purpose of lubricating and cooling the injectors and pump.
Clean out all traces of debris and crud from your bulk storage fuel tank each and every year. It’s a small task considering the savings it will potentially lead to.
Ensure you have the correct filtration on your bulk storage tank — a breather filter on the inlet, a two micron filter on the outlet and a water trap, at a minimum.
Don’t try to skimp and save on filters, buy genuine and change regularly. They are a very small cost in comparison to expensive repairs and downtime. A €20 filter doesn’t look so bad now to the guy forking out €4,000 to have his entire system overhauled.