If you are not a diesel mechanic or have not had much exposure to diesel engine technology, you may not know that a), diesel engines with common rail fuel injection can be up to twice as efficient as petrol engines, and b), what to tell a customer who is complaining about his diesel vehicle becoming progressively less “peppy”. In many such cases, there may not be obvious signs that something is wrong with the vehicle, or with any particular control/management system, so in this article, we will take closer look at how, and why diesel engines can progressively become less efficient over time, starting with this question-
Even though diesel fuel has a significant energy density advantage over petrol, less than 50% of a diesel engine’s overall efficiency derives from this. In practice, about 60% percent derives from a combination of a diesel engines’-
In addition, all of the above combine to make diesel engines run significantly cooler than petrol engines with comparable displacements, which further increases a diesel engines’ efficiency because less of the fuel’s calorific value (as compared to petrol’s calorific value) is converted into heat that is shed to the atmosphere without having done any useful work.
However, the downside is that a diesel engines’ superior efficiency as compared to a petrol engine with a similar displacement can only be maintained when all the factors that make a diesel efficient in the first place are all operating at optimum levels. In fact, any single factor, system, or control function that controls, regulates, and/or influences diesel combustion that does not function optimally can potentially cause a diesel engine to lose progressively more power, and to become less efficient as time passes and mileage increases, so let us start by looking at-
Even though the quality and purity of diesel fuel in Australia leaves much to be desired, old-style unit injectors are not, or were not as susceptible to damage caused by poor fuel quality as modern common rail injectors are. The main reasons for this are the facts that unit injection pressures were much lower than common rail injection pressures, and that unit injectors had relatively large injection orifices. In practice, vehicles running with unit injection systems and secondary fuel filters that filtered down to about 5 microns were relatively immune to injection orifices becoming clogged, because most solid contaminants simply passed through injection orifice without becoming stuck.
However, things are vastly different on common rail injection systems. Typical injection pressures on these systems can be as high as 2000+ bar, and up to six injection orifices (that range between 120 and 200 micron in diameter*) per injector mean that fuel can be vaporised more efficiently than was ever possible to do with unit injection systems. Moreover, multiple injection events per compression stroke also improve bulk mixing of the fuel and air, which greatly improves the combustion process.
*By way of comparison, human hair is on average, about 100 micron in diameter.
It is important to remember that the primary driver of improved combustion in diesel engines is the need to reduce emissions. While common rail diesel engines are orders of magnitude cleaner than ever before, the downside for us as technicians is that any deviation, no matter how small, from optimum fuel atomisation can cause misfires, power loss, increased emissions and decreased fuel economy- not to mention customers who demand answers we may not be able to give them, or at least, not immediately.
Let us use as a practical example, a modern common rail diesel injection system on which dirty fuel had clogged one injection orifice. If all the injection orifices were clean, the finely atomised fuel would be sprayed into the air charge in at least six directions, which greatly improves the distribution of the fuel throughout the combustion chamber. When ignition of the fuel occurs, it occurs simultaneously at multiple points* throughout the air fuel mixture, and if the system uses multiple injections, the overall combustion event is improved even more. The result is a phased and almost perfect combustion of the fuel, which is very difficult, if not impossible to achieve with unit injection.
*This is different from ignition in petrol engines, where ignition generally occurs at the spark plug, and then propagates outward from the point of ignition.
At this point, it hardly matters how and why injection orifices on common rail diesel injectors become blocked. What does matter however is the fact that one clogged orifice prevents the even distribution of the fuel throughout the combustion chamber. The practical effect of this is that a), the air/fuel mixture does not ignite uniformly, and b), that non-uniform combustion causes the detonation flame front to develop and/or propagate in a chaotic manner, which results in uneven and incomplete combustion. A further effect is that if the system uses multiple injections, the chaotic combustion pattern could prevent the ignition of fuel injected during the last one or more injection events, which brings us to-
How to recognise clogged injectors
Typical symptoms of clogged injectors include, but are not limited to, rough running at idle, visible smoke from the exhaust, increased fuel consumption, clogged or damaged catalytic converters and/or DPF’s (Diesel Particulate Filters), frequent DPF regenerations or a failure to initiate DPF regeneration. This last symptom is hugely affected by how the vehicle is used, and it may sometimes be necessary to obtain live data on exhaust backpressures before performing a forced DPF regeneration to either confirm or eliminate a clogged DPF as the cause of power loss.
However, knowing that Australian diesel fuel is several orders of magnitude dirtier than it should be, a diagnostic procedure for issues like power loss and efficiency that gets progressively worse should ideally follow the following pattern to save time-
The steps outlined above will usually resolve the issue in nine out of every ten cases, but if the problem persists or gets worse, you need to consider-
If all the factors that make a modern common rail diesel engine work efficiently are equal, then compression pressure would the first among equals.
Since compression pressure is the triggering mechanism in diesel combustion, it follows that a steady decline in effective compression pressures over time would have a direct bearing on how well (or otherwise) the air/diesel fuel mixture will combust as the engine ages. In fact, most diesel engines in modern light vehicles will begin to show a marked decrease in power and efficiency when effective compression pressures decrease by as little as 5%- and in some cases, considerably less than 5%.
Nonetheless, when considering compression pressures on diesel engines it is important not to get hung up, or to be distracted by the relationship between the volume and pressure of the intake air. Since diesel engines are not throttled like petrol engines, diesel intake systems between the turbocharger outlet and the intake valves are fairly uniformly pressurised, and therefore, a diesel engine simply takes in as much air as is available during the time the intake valves are open. As a practical matter, this eliminates inaccurate airflow readings through faulty MAP or MAF sensors, as well as the need to base injector pulse widths on intake manifold pressures, among other things.
So, why is this important? It is important because the only thing that can really affect the relationship between engine power/efficiency and compression pressure is how well (or otherwise) the piston rings contain the compression pressure from the time it begins to build, to the time the piston begins its downward travel after combustion had taken place.
Thus, if we assume that the exhaust valves are not leaking, compression pressure can only escape past the piston rings. While mechanical wear of the piston rings/cylinder wall interface is inevitable, the role that proper lubrication plays in the rate of wear is much more important on diesel engines than on any petrol engine. There are several reasons for this, but the most pertinent reason is the fact that the tribological* loads on piston rings are much more severe in diesel engines than they are in petrol engines. Let us look at this issue in more detail-
*Tribology is the science and study of the principles of lubrication, friction, and mechanical wear of surfaces that are in relative motion, such as for instance, piston rings that slide up and down in cylinders while pressing on the cylinder walls at the same time.
While it is true that the primary motion of pistons are aligned with the axis of the cylinder bore, all pistons also exhibit slight side-to-side motions that are imparted to them by the rotation of the crankshaft. These motions vary relative to the pistons’ position in the cylinder, as well as with engine speed and load. Moreover, while the effectiveness of cylinder sealing is greatly enhanced by gas pressure that effectively pushes the ring face onto the cylinder wall, the slight lateral motions of a moving piston bear directly on the thickness of the lubricating film between the ring face and the cylinder wall.
The very high temperatures that obtain near the top end of a piston’s travel tends to heat the oil film on the cylinder walls to the point where the oil’s viscosity is greatly reduced, which makes it relatively easy for the leading edge of the top piston ring to literally “wipe” the oil off the cylinder wall. Essentially, the top ring then operates under momentary boundary lubrication conditions, since much of the width of the ring is in direct metal-to-metal contact with the cylinder wall.
However, most modern diesel engines are designed to be capable of enduring the increased rate of mechanical wear of the top end of their cylinder walls for several hundred thousand kilometres without adverse effects, but for the most part, this is only true when diesel engines are not running on dirty, degraded, unsuitable, overly diluted, or substandard engine oil.
Note though that since the second ring is generally not affected to the same extent as the top ring, detecting this type of mechanical wear is generally not possible without dismantling the engine, since the cranking speeds at which compression tests are typically performed are too low to induce lateral motions in the piston.
NOTE: While a cylinder leak down test might reveal worn piston rings on a diesel engine, you need to compare the leak down rate with reliable service information if you want to remove the guesswork from the procedure.
In practice, this means that since both gas pressure behind the top ring and piston speed are relatively low at cranking speeds, the top ring is typically not forced into a position where much of the ring face is lifted off, or moved away from the cylinder wall by excessive lateral motions. As a result, both the top and second rings typically perform well at cranking speeds, which usually yields a good compression value- albeit an often-misleading compression value that may not bear any relation to actual compression values at higher engine speeds.
Consider the image below-
The detail in this image has been exaggerated for clarity, but it shows how the lateral motion of a piston can scrape off much, if not the entire thickness of the oil film from a cylinder wall. In engines that already show a marked degree of cylinder wall wear, the top ring operates under boundary lubrication conditions for significantly longer than in an unworn engine, which accelerates wear of both the top ring and the cylinder wall even further. Note that “LOC” stands for Lubricating Oil Consumption”.
Based on the above, and in cases where it is known that a customer has not kept up with scheduled services and oil changes, it is almost certain that his engine’s gradual loss of power and efficiency is due solely to a loss of compression pressure past worn top piston rings. Except for rebuilding the engine there is no cure, but this type of issue can largely be avoided by advising customers not to skip scheduled oil changes and always to use the best oil available- as opposed to the best oil they can afford.
Due to how engines are constructed, there is no way to avoid or eliminate lateral motions of pistons. However, since the oil film on cylinder walls are typically only about 10 microns thick at mid-stroke, and often less than 5 microns thick at both TDC and BDC, using the recommended oil in diesel engines offers the best possible protection against the effects of piston rings operating under boundary lubrication conditions, which brings us to-
Fortunately, there are other, less serious causes of power loss and reduced efficiency in diesel engines, which could include one or more of the following-
Intake system leaks
Since diesel engines perform best when full boost pressure is present, there is an almost direct correlation between the degree of power loss and the size of a hole, leak, or perforation in the intake system. While leaks can occur almost anywhere, the most common leak sites are unsecured intake duct connections, or perforations in intercoolers.
Intake system restrictions
This hardly needs explaining; check the condition of the air filter element, and make sure that no workshop rags, rodent and/or insect nests, or other impediments to air flow are present anywhere in the intake system.
Poor turbocharger performance
Common issues with turbochargers include worn compressor wheels, leaking waste gates (dump valves) that allow boost pressure to escape, leaking exhaust manifolds that allow drive pressure to escape, or serious imbalances in either or both the turbine and compressor wheels that prevent the rotating assembly from spinning at optimum speeds.
Excessive free play or mechanical wear in valve trains
Since diesel engines depend on compression to work any free play or wear in valves, valve lifters, camshafts, rocker arms (tappets), timing belts/chains and associated sprockets, pulleys, tensioning devices, and/or guides can conceivably affect the volume of air that each cylinder takes in. Reduced intake volumes are exactly analogous to increased combustion chamber volumes, and if the deviation is big enough ignition may not occur, or if it does, the combustion process may not be complete, thereby affecting engine power and efficiency.
Excessive exhaust backpressures
Any restriction in a diesel exhaust system that causes exhaust backpressure to rise beyond maximum allowable levels will interfere with exhaust gas scavenging from the cylinders, which will in turn, affect how the engine breathes. Typical restrictions include clogged or damaged silencers, DPF’s, and particularly SCR (Selective Catalytic Reduction) catalytic converters. Note though that exhaust restrictions will usually (but not always) trigger fault codes and/or warning lights, so be sure to extract and resolve all stored fault codes before condemning expensive exhaust components out of hand.
There was a time when diesel engines would, and did run on almost anything that burns, but this is no longer the case. Modern diesel engines will only run efficiently when they are in excellent mechanical condition, and all control parameters are within acceptable limits. However, when you diagnose driveability issues on diesel engines, it is important to a), think in terms of how diesel combusts, and b), to remember that fuel trims play no role in diesel engine operation.
Essentially, your thought processes have to undergo a paradigm shift. From the complex relationships between the density, temperature, and flow rate of the intake air and how those relationships affect petrol combustion, to the simple fact that a modern diesel engine only requires finely atomised fuel and compression to work. You will find that once you make this paradigm shift, diesel engines will lose all of their mystery and diagnosing driveability issues will become a proverbial walk in the park.
