Diesel performance is nothing new. What is new is the level of interest in diesels for all kinds of racing; everything from traditional truck and tractor pulling to drag racing, even sports car and endurance racing.
Diesels are setting all kinds of records from land speed to elapsed time. So if you’re not already dabbling in diesel performance, you may be soon.
Diesel engines are a different type of animal when compared to gasoline engines. While both use the same four-stroke cycle of combustion, diesels use the heat of compression rather than a spark to ignite the air/fuel mixture. A diesel engine requires a much higher static compression ratio than a gasoline engine, and on most engines there is no intake vacuum because the engine is unthrottled. Speed and power outputs are determined by the fuel injection system.
Most diesel engines are also turbocharged, yet make most of their torque and power at relatively low rpm (typically 2,500 to 4,000 rpm). However, in some forms of racing, diesels are being revved to 5,000 to 7,000 rpm).
So what does this mean to you? It means if a customer asks you to build a performance diesel engine, it’s going to take a different approach than building a performance gasoline engine. The bottom end in most diesel engines is fairly stout already, with forged steel crankshafts and beefy connecting rods, so the amount of modifications that may be needed here are usually minimal. And, because most street performance and pulling applications don’t require a lot of rpms, engine balance is not as critical as in a high revving gasoline engine.
Pistons may have to be cut, modified or replaced, however, depending on how much turbo boost the engine will be running, what kind of fuel it will be burning (diesel only, or diesel plus nitrous oxide or alcohol), and what kind of modifications are being made to the camshaft and valvetrain, which is the focus of this article.
For street driven daily drivers, you want a camshaft that delivers a lot of cylinder pressure – which means limiting valve overlap.
Breathing Issues
Because diesels require a lot of compression, camshaft duration tends to be short with minimal overlap. Valve lift may also be limited by the tight piston-to-valve clearances in most diesel engines. So unlike gasoline engines, you can’t go hog wild with lift and duration to make more power. A bigger camshaft can provide more power, but only if the lift and duration are right for the turbochager, cylinder heads, pistons, valvetrain and application.
The most significant power gains in a diesel engine come from increasing the amount of boost delivered by the turbocharger(s). But there’s more to the secret than just bolting on a bigger turbo. The camshaft must have the right exhaust characteristics so it will spool up the turbo faster and keep it spinning at peak efficiency in the engine’s power band. For some types of pulling, the power band can be quite narrow, say only 2,000 to 3,000 rpm for a big agricultural diesel engine, or maybe 3,000 to 5,000 rpm for a pickup truck. A street truck, by comparison, may make most of its power from 1,800 to 3,200 rpm.
If engine speed drops too low during a truck or tractor pulling event, the turbo can stall or “chirp,” causing the boost pressure to suddenly drop. This kills power and may even cause the engine to stall.
“Less is more” is often the best advice when choosing a diesel performance cam, especially if the engine is going to be in a street vehicle.
Choosing a Cam
Always talk with the customer to find out exactly what they want to accomplish. Also, find out what kind of turbo, boost pressure and other modifications are being made to the engine so the cam grind you provide is the best one for their application.
For example, is it a street/towing application? Will they be sled pulling or drag racing?
These are important things to know when choosing a performance cam.
For street-driven daily drivers, choose a camshaft that delivers a lot of cylinder pressure — which means limiting valve overlap. You’ll want adequate separation in valve timing, but you also have to keep the lobe separation tight so you don’t run into clearance problems.
Remember, you don’t really need a lot of valve lift at low rpm on the street. Increasing lift often requires cutting piston reliefs or changing pistons.
Many diesel specialists say most diesels don’t require a lot of camshaft duration, and that different profiles for the intake and exhaust lobes deliver the best performance. In fact, one of the biggest changes seen in recent years is that improvements in turbocharger design are allowing us to use more camshaft (more duration). Turbos that were capable of producing 35 psi of boost five years ago have been replaced with turbos capable of making 60 psi of boost today.
These kinds of changes mean engine specialists don’t have to rev the engine higher to make more power — all they need to do is optimize the boost delivered by the turbo with the right sized cam. A camshaft with asymmetrical profiles for the intake and exhaust valves will deliver a broader and fatter power band.
Most diesel engines, with some exceptions such as Duramax and some late model Caterpillar and International engines, still use flat tappet camshafts with flat bottom valve lifters.
Valvetrain Considerations
Since most diesels run at relatively low rpms, modifications that are required for the valvetrain are often minimal. This means you don’t need monster springs and pushrods to prevent the valves from floating at high speed. On the other hand, there may be some concerns about the effects of turbo boost on the intake valves.
Many diesel engines run very high boost pressures, from 80 to as much as 200 psi on some really wild racing engines. So there’s a concern that really high boost levels may overcome the force exerted by the springs on the intake valves forcing the valves open. While some diesel performance experts say that for every 10 psi increase in boost, you should use springs that are at least 10 psi stiffer than the stock springs (closed seat pressure), others say this really isn’t necessary because when the piston is coming up on its compression stroke, the pressure developed inside the cylinder will hold the intake valves shut.
On Cummins engines, there is also a concern that overly stiff valve springs may increase wear in the cam bores. In Cummins engines, there are no cam bearing inserts — the camshaft journals run directly on the machined bores in the block.
Because of this, extremely heavy valve spring pressure pushing the rocker arms, pushrods and lifters down against the camshaft may cause the bottom of the cam bores to wear out. As the cam settles lower and lower into the block, it upsets valve lift and timing, causing a significant loss of power. One way to prevent this in a performance Cummins engine with stiffer than stock valve springs is to machine out the cam bores in the block and install aftermarket
bearing inserts.
Lubrication Issues
Another concern with increasing valve spring pressure is camshaft lobe and lifter wear. Most diesel engines, with some exceptions such as Duramax and some late-model Caterpillar and International engines, still use flat tappet camshafts with flat bottom valve lifters rather than roller lifters.
The reasons for this are both economical and practical. Since most stock diesel engines run at low rpm and have relatively mild valve springs, there is not a lot of pressure on the cam lobes. Consequently, less expensive flat bottom lifters are usually fine to use. But when a diesel engine is modified for performance (and stiffer valve springs are installed to allow higher engine speeds), the increased load may cause rapid cam lobe and lifter wear — unless a racing oil or crankcase additive that contains an adequate level of ZDDP (Zinc Dialkyl Dithio Phosphate) is used.
In recent years, ZDDP levels in motor oil have been sharply reduced to prolong the life of the catalytic converter. Historically, ZDDP has been added to oils in concentrations of around 0.15% for phosphorus and 0.18% for zinc. But when “SM” rated motor oils appeared in 2005, phosphorus levels were reduced to 0.08%, or roughly half that of the previous generation of motor oils.
The same problem affects both gasoline and diesel engines that are still using flat tappet camshafts. The remedy is to coat the cam lobes with high pressure assembly lube, and to use a crankcase additive or racing oil that contains adequate levels of ZDDP. Or, convert the valvetrain from flat tappet to a roller cam and lifters (if available).
Ford’s 6.0L Powerstroke diesel is a powerful torque monster, celebrated by Ford enthusiasts everywhere, but like every engine, it is not without its common issues and factory limitations. In this class, Saving Your 6.0 Powerstroke, Ron Bilyue dissects the common inherit problems with the 6.0L Powerstroke and how to fix them using enhanced aftermarket parts utilized in the performance industry. Don’t just replace parts with the same thing from the factory that failed in the first place, upgrade and never worry about that component failing again. These parts will ensure a longer life than Ford factory replacement parts that caused the issue in the first place.
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Trying to make meaningful comparisons between engine types can be confusing. Aside from the common usage of two (or more) units of measure for each specification, there are often multiple ratings for each engine model. In many applications, such as marine and generator sets, special ratings are offered that are intended specifically for a particular usage. These special ratings usually have limitations placed on them.
The “metrification” of units of measure seems to be almost universal. Most specification sheets show both S.I. and older SAE measures. There are many readily available conversion tables that provide easy translation of the common units used to describe diesel engine specifications. While it is not the intention of this writing to serve as a conversion chart, a few common units of engine measure are converted below.
HP = horsepower
kWm = kilowatts (mechanical)
N·m = Newton metre
Note: It is important to use the same basic criteria when making any comparison. For example, as long as each engine’s fuel consumption uses the same weight per unit of fuel the comparative performances will be meaningful.
In spite of “metrification”, diesel engines are often referred to by their horsepower outputs. The metric comparison is kilowatts (kW). Until recently, in North America, engine output was measured in horsepower and electrical output in kilowatts. This can lead to some confusion when the engine is required to drive a generator set. The mechanical kilowatt output of the engine (kWm) does not allow for efficiency losses in the generator, or perhaps other parasitic losses such as a cooling fan, before the generator’s electrical output, measured in electrical kilowatts (kWe). Kilowatts (electric) is the power available at the generator terminals. As with engine power ratings, there may be three or more generator ratings (continuous, prime, and standby) dependant upon the intended usage of the machine.
There are several common methods of rating industrial and marine diesel engines. It is not unusual to see five differing power output ratings for the same engine model. Ratings may be performed to “DIN”, “SAE” or JIS (three governing bodies) standards. Basically, the most important factor is the intended usage of the engine. Determine which rating you need to use for the intended service of the engine and ask for the HP or kWm rating of the engine to be expressed in the most suitable terms. All manufacturers who offer multiple power ratings also offer guidelines for the usage of the engine at the various power ratings.
An example of this would be illustrated by the following sample curves. The horizontal axis shows engine speed in revolutions per minute (RPM) and the vertical axis indicates power in both kWm and in HP. There are two curves shown and the testing standards are indicated (ISO 3046). Typically these curves are referred to as “continuous” (the lowest output), and “intermittent” (the upper curve).
Given equal or better conditions than the test conditions for fuel, ambient temperature and altitude, this engine would provide the user with any of the power outputs shown at any of the speeds shown.
Probably nowhere can more “elastic” ratings be found than for high-speed marine engines. One reason for this is that the usage of marine diesels can vary from full power, “24/7” to very intermittent service, high speed vessels. As well, with marine engines it is possible to use the seawater for after-cooling the turbocharged intake air, allowing more fuel to be efficiently burned. Marine, like any other usage, requires the buyer to clearly specify the nature of the vessel’s usage.
So why not buy the greatest available horsepower from a given engine? The answer is engine life (for more information on engine life, see How Long Will a (Marine) Diesel Engine Last?). Engines have a design life at a given power output. The service factor of the application must be accounted for when selecting the engine. For example, water pumps may be required to operate at a given power output for long periods of time. This is a “continuous” application. A brush chipper, normally, only works hard for brief periods while material is fed through the blades. This is an intermittent duty. There are special applications as well. Fire pumps, high-speed emergency vessels and the like may have curves that are for use in their particular application only.
Engines are initially designed to perform a service or some range of services. For example, “automotive” engines are typically compact, lighter weight engines designed for use in vehicles. The engine may use lighter, less robust components than an engine designed for use in heavy equipment or commercial marine applications. Automotive derivative engines may still offer a “continuous” horsepower rating but the design life of the engine may be significantly less than for a heavier industrial style engine. The most common “clues” for durability are the cubic displacement of the engine and the RPM at which it develops its horsepower.
There is no right or wrong in this. If the application is vehicular an automotive style engine is built for the purpose and should provide adequate life. If the application were heavy industrial an automotive style engine would be misapplied and would not provide a reasonable service life.
There are tables that provide guidelines for engines used in various services. Engine manufacturers publish application information and many equipment manufacturers also provide information on input power requirements. This information and an idea of the number of hours of service required helps to define what engine is “right” for the job.
It is a trade-off between power output and the service life of the engine. The key to a satisfactory experience with an engine is to define the intended usage and select an engine and a power rating that should provide the hours of service required.
Horsepower is a rate of doing work. Torque is “the rotary force in a mechanism” according to a dictionary definition. The two are related (Torque lb-ft = H.P. X 5252 / RPM) but torque is often misunderstood. Since there is a fixed relationship between HP (or kWm) and torque, two engines having the same horsepower at the same RPM will have the same torque. However, at work, the two engines may perform very differently. The reason for this is they may have very different torque rises. Therefore they respond differently to the demands of the load.
The length of piston stroke, number of cylinders, rotating mass and other factors, affects torque rise. Newer electronically controlled engines are able to produce torque characteristics that could not be achieved with mechanical fuel controls.
Engine horsepower curves often show the torque curve or “pull down torque” as well. This curve illustrates the amount of torque available from the engine as a load is applied that exceeds the engine’s rated torque at the operating RPM. The difference in the torque at the rated RPM and the maximum or peak torque is the “torque rise”. It is usually expressed as a percentage. (Peak Torque – Rated Torque / Rated Torque = Torque Rise X 100)
In this case the rated torque is 477 lb.-ft. and the peak torque is 657 lb.-ft. @ 1200 RPM. The torque rise is: 657 – 477 divided by 477 = 38%.
Note that although there are two horsepower curves, continuous and intermittent, only the intermittent torque curve is shown. The assumption is that if the engine is “pulled down” (reduced in RPM by the load), then the engine will be performing on its intermittent rating curve. Torque curves are usually available for any published horsepower rating.
What all this means in actual usage is that, in many applications, the engine with the greater torque rise will do its work more quickly. It will seem more powerful and responsive. This difference will be very evident in applications where the engine is routinely pulled down from its rated speed by the load. Examples of this are a drilling rig lifting the rod string, a grinder processing a stump or a loader digging into a rocky bank. Even applications not generally thought of as sensitive to torque rise such as generator sets and marine engines can, under some conditions, benefit from good torque rise characteristics. (Pulling a heavy trawl net, bucking a current or starting a motor load, for example.)
Torque and torque rise are very important considerations in many applications particularly those where the engine is routinely pulled down from its rated speed owing to the effects of the load. Greater torque rise allows the engine to run at a higher r.p.m. under load and thereby accomplish its work more quickly. In extreme conditions, inadequate torque rise will prevent the engine from accepting the load and it will stall.
There is a lot to add on the importance of torque. “Droop” and “isochronous” governing, electronic or mechanical controls and other factors enter into the aspects to consider. Again, if the intended usage is clear, the best option will usually be apparent.
For good reason, people often want to know how much fuel their engine will consume, Fuel is by far the greatest cost over the life of most engines. A seemingly small difference in fuel consumption can provide a large saving of the course of an engine’s lifetime. There are applications where the costs of an engine or a generator set can be returned over a relatively short period of time simply by sizing the unit correctly for the load.
Engine manufacturers publish fuel curves. These curves are usually certified to be correct, within narrow limits, given certain factors such as minimum cetane rating and acceptable ambient conditions. Fuel consumption testing is usually done by weight of fuel consumed, over a known period of time at a known power output. The test results may be published by weight or they may be converted into volume.
Many manufacturers also publish partial load fuel consumption data. This can be very important information, as most engines do not run at their full power rating all the time. Owing to efficiency losses during combustion, partial load fuel consumption is not “linear”. That is, a 50% load will not consume 50% of the full load fuel consumption. In some cases the differences in partial load fuel consumption figures can be quite dramatic when compared from engine model to model.
Modern diesel engines, especially electronically controlled engines, are very efficient in terms of usable energy produced for the fuel expended. However, any engine has to be correctly applied in order to provide economical and reliable service.
The only way to accurately predict fuel consumption is to know what the load on the engine will be. Sizing the engine correctly is critical. Too much power for the load will result in poor fuel economy (and other problems). Too little power will result in a shortened engine life.
In many applications a fuel consumption curve can be used to help find the most economical source of power.
Here is an example of what one might find when looking at fuel consumption curves.
Chart ‘A’ shows a “Heavy-Duty” horsepower curve for an engine rated 250 h.p. @ 2200 RPM. Chart ‘B’ illustrates a horsepower curve for an engine with a rating of 225 h.p. (continuous) at 2400 RPM.
If we wanted to drive a water pump requiring 220 h.p. we could operate engine ‘A’ at 1600 RPM and select a water pump with an impeller trimmed to suit that RPM. The engine could be expected to consume:
– 220 h.p. x 0.32 lb = 70.4 lb/hr or 9.86 US gallons (37.47 litres)
Engine ‘B’ would have to operate at 2200 RPM to provide us with 220 continuous h.p. Its fuel consumption would be:
– 220 h.p. x 0.35 lb = 77.0 lb/hr or 10.85 US gallons (41.23 litres)
The difference in fuel consumption is only 0.03 lbs per horsepower per hour. However if the pump operated for 2500 hours in a year (48 hours per week) the fuel savings would be 2,475 US gallons (9,405 litres). Over the expected life of the engine, this represents a huge saving in operating cost.
Staying with our example of ‘A’ and ‘B’, one would expect that to produce same continuous horsepower at a lower engine speed would require a larger displacement engine. So the initial purchase price of the engine and its accessories (radiator or other heat exchanger, air cleaner, muffler, etc.) would be higher. However when selecting an engine, fuel consumption costs should be considered. This is especially true in applications required to run long hours in areas where the cost of fuel is particularly high.
Many applications allow the engine to be operated, under normal circumstances, at a pre-selected engine RPM. It can pay big dividends to select an operating speed where the engine delivers its best fuel economy.
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