Diesel engines are going through a dramatic and rapid change in technology and for this reason they are becoming quieter and more responsive and are quickly becoming popular among consumers. This is why an entire chapter has been dedicated to the diesel engine. When diesels first evolved beyond the the early versions poorly built into General Motors passenger cars in the 1980s, they were naturally aspirated and placed into light trucks like the Chevrolet and Ford 2500 and 3500 series pick-ups using the 6.2 GM and the 6.9 and later 7.3 diesel engines respectively. Several companies developed turbo kits for these engines because owners using these early diesels bought them to do legitamate work such as hauling RVs or other ligh-duty work trailers. Unlike the changes to gasoline engines during the ’70s and ’80s, the technological improvements now available on diesel engines not only make them more consumer friendly, but these same design improvements make them more powerful and responsive which attracts consumer expectations for all around driving pleasure. Consumers have found out that hot rodding a diesel is much easier than a gasoline engine due to diesel fuel’s inherient characteristic of not being limited by the octane rating considerations in gasoline. While there are many brands of easy to apply fuel and timing retrofit control modules, the turbocharger quickly becomes the most limiting factor in raising the performance expectations in your consumer diesel.
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It’s fair to say that the development of turbochargers is mostly due to the diesel engine. Most of today’s advanced performance automotive turbocharger compressors and turbines were originally developed for various diesel applications but have been adapted to specific configurations for compatibility with gasoline engines for the performance community.
If you own a diesel-powered vehicle: congratulations! If you’re contemplating the purchase of a diesel, you are wise and not alone. It’s not that diesel is better than gasoline or vice versa, each engine has its strengths and weaknesses. But diesels do have some inherent advantages.
First off, the diesel engine is the most thermodynamically efficient form of internal combustion engine that’s ever been mass produced. The diesel will typically get 30 to 40 percent greater fuel economy than a gasoline engine of the same horsepower. Diesels typically last much longer and are more reliable because they typically run lower exhaust temperatures and are more heavyduty in terms of their construction. The extremely high torque of the diesel, along with its fuel efficiency, is what makes it a perfect work engine and is why commercial vehicles are dominated by diesel engines. All these advantages have also proven beneficial in certain forms of endurance racing such as the 24 hours of LeMans.
Prior to 1992, the sulfur content in diesel fuel in the United States was not regulated. Late that year, an EPA regulation stated that sulfur content in on-highway diesel fuel shall not exceed 500 parts per million. Then in January 2007, the regulation tightened and stated that the sulfur content shall not exceed 15 parts per million. This 97 percent reduction in diesel fuel sulfur content is referred to as the great enabler for diesels. The 2007 change in the sulfur content in diesel fuel is similar to the impact of removing the lead from gasoline many years ago.
Now that sulfur is practically eliminated from diesel fuel, the sulfuric acid that was present in diesel exhaust is now so low that exhaust after treatments on diesel vehicles can compliment the advanced fuel injection systems and variable geometry turbocharging to make today’s diesel run as clean, or cleaner, than gasoline engines—all while lasting longer and getting much better fuel economy. Further, the use of exhaust gas recirculation (EGR) significantly lowers the nitrous oxide emissions.
In Europe over 50 percent of all new consumer vehicles are diesel powered. By contrast, of the approximately 15 million consumer vehicles sold in North America in 2007, only about 3.6 percent, were diesel powered. Historically, consumer diesels had a bad image in this country due to the smell, the smoke, and the characteristic diesel rattle. Further, the early GM passenger car diesels gave many consumers a bad view of diesels due to the problems those early engines encountered. The price of fuel in the United States has been about the cheapest in the world as well. In Europe they would pay for a liter about the same amount Americans would pay for a gallon up until about 2003. This dramatically higher cost of fuel drove European consumers to look at more fuel-efficient diesel engines. The diesel engine technology that developed by way of market demand and emissions standards has now all but eliminated the smell, smoke, and diesel rattle, but many consumers are unaware of these facts.
The current diesel revolution in America is lead by the light-duty diesel pickup truck. Ford, General Motors, and Dodge all have excellent diesel pickup trucks. Since about 2004, nearly 75 percent of all 3/4-ton and above rated pickup trucks sold have been diesel powered. The sport truck boom of the 1990s ended, and the diesel hot rod market was born.
Today diesel pickup trucks are virtually a status symbol. Gone are most of the historical consumer objections to diesel. Diesel rattle was caused by the shock wave from the violent burn of diesel fuel as it is injected into the engine. Today’s diesel is much quieter due to a fuel system design feature known as pilot injection. Pilot injection is where a very small amount of fuel is injected into the combustion chamber just a few degrees of engine crankshaft rotation before the main injection event. There may be one or more pilot events, which make a smoother and more controlled burn of the main injection. Following the main injection event there may also be one or more post injection events that help to lower emissions, too. This precise control over fuel management is done using the new High- Pressure Common Rail Fuel injection system that also uses rapid-fire piezotype electronic injectors. This combination of new technologies is not something so futuristic that it’s only for racing vehicles; this type of injection system is actually in production on many new vehicles right now!
Smoke is also gone, thanks to these new high-pressure fuel systems and the advances in applying variable geometry turbos. Since diesels have become more popular more refueling stations have added diesel to their offering. One of the big concerns of alternate fuels has been the refueling infrastructure. I maintain that the oil companies can, and will, add diesel to their networks just as fast as the market demands. The challenges of adding diesel fuel to a gas station are child’s play as compared to compressed natural gas, propane, or other proposed alternate fuels.
Many consumers are still not aware of where diesel technology is today and that’s understandable. The rate of technological development exceeds public awareness. But there are many forces that are affecting this change in awareness. Who would have thought just 10 years ago that there would be a Diesel Hot Rod Association (www.DHRAonline.com), a slew of magazines specifically targeting diesels enthusiasts, the world’s fastest sport truck as a diesel (Project Sidewinder), or even a diesel talk show in webcast format (www.DieselRadio.com). Yeah, if you’re an engine guy, there’s a diesel in your future. Especially if you become a successful racer, that’s what will get you to the track. Besides, you don’t want to have to sit out of the up-hill races that teams get into between racing venues with their tow trucks, I never did!
On-Highway Performance Diesel Roots
The commercial diesel engine can be, for the most part, credited with driving the development of turbochargers from World War II to where they are today. During the postwar era, the diesel engine’s development grew as American industry applications of diesels grew. The Caterpillar Tractor Company was instrumental in driving the development of the modern era turbocharger to allow higher horsepower engines to be used without the sacrifice of dramatically increasing engine displacement and the resulting costs these larger engines would create.
As the turbocharger matured, it’s flow range improved, manufacturing costs lowered, the number of moving parts dropped, and durability improved. As fuel economy and horsepower demands grew, heavyduty diesel engines made by Cummins, Detroit Diesel, Mack, and Caterpillar, became more refined and durable. The turbocharger’s development played a significant role in this evolution as well. While Caterpillar was an off-road market leader, the Cummins Engine Company of Columbus, Indiana, developed an on-highway commercial dominance in heavy truck applications through the 1970s.
By the mid-’70s, most heavyduty diesels were turbocharged. Advancements in turbocharging, however, could be applied backward to older engine designs, known as diesel turbo retrofitting. Schwitzer, then a division of the Wallace Murray Corporation, entered the aftermarket with a turbocharger retrofit kit that allowed owners of older nonturbocharged Cummins engines to be upgraded with a turbocharger. This proved to be quite successful and Schwitzer sold many turbocharger retrofit kits for the older Cummins engines.
By the early ’80s, Schwitzer introduced the 4LHR-model turbocharger. The Schwitzer 4LHR was designed to have the same installation envelope as the original equipment turbocharger model made by Cummins at that time. The 4LHR contained design features that produced slightly different engine performance that made it a popular choice for Cummins diesel engine owners interested in upgrading their turbocharger when the original Cummins designed turbocharger required replacement. This was an important historical event that set the stage where engine performance could be enhanced by simply upgrading the turbocharger alone.
Turbocharger development was fairly rapid during that time frame due to advanced electronic instrumentation, and computerization compressed the development time to advance the art. If we take a close look at the design differences between the original Cummins VT-50, ST-50, and T-46 model turbos available during that era, versus the Schwitzer model 4LHR, we can better understand the important functional performance aspects of different turbocharger designs. These differences, and how they affect performance, are important whether your interest is in gasoline or diesel. Let’s analyze the cutaways of each unit and explore the internal differences. At a quick glance both look similar, but there are some very important design differences.
First, note the compressor wheel in each turbocharger. The Cummins T-46 design used a straight radial wheel design, while the Schwitzer 4LHR used a backward curved impellor (BCI) design. This can be seen by closely looking at the compressor wheel tip in each wheel. The straight radial wheel blades run perpendicular to the shaft all the way to the tip, while the BCI design blade tips are swept backward opening up the tip and allowing air diffusion to begin as the air exits the wheel and enters the parallel wall diffuser. Here it turns to static pressure before it enters the larger volume compressor volute in the 4LHR design.
The BCI wheel produced a higher efficiency and broader flow range than the straight radial wheel. For this reason the Schwitzer catalog cross-reference was written by matching turbine flow ranges because one 4LHR turbocharger would replace several Cummins part numbers that used many different compressor wheel and cover combinations within a horsepower range. For example there may have been dozens of Cummins turbo model part numbers for various horsepower and torque rise ratings as compared to only one Schwitzer part number within a range of 270 to 335 horsepower. Another 4LHR model covered the 335 to 370 horsepower range, and only a third part number covered the 370 to 400 horsepower range. Throughout this wide range of horsepower ratings there were literally dozens of Cummins part numbers.
This could only be commercially achieved through the use of the broad flow range of the BCI wheel design. This is also why this same type of compressor wheel design is used almost exclusively in both gasoline applications and light-duty consumer diesel applications where an engine is operated across a wide operating range of engine RPMs.
As we continue our analysis, the next most obvious design difference is the bearing systems. The bearing systems have many jobs, but a commonly overlooked aspect is the impact of the turbo’s efficiency as a machine. Bearing drag is a “takeaway” from the turbine energy that drives the compressor. To help illustrate this point, imagine holding onto a shaft driven by a turbine that is trying to turn a compressor. As you squeeze your hand to tighten your grip, you increase the resistance against the turbine shaft. To compensate for this increased resistance, a smaller turbine could be used to raise exhaust pressure, which in turn further restricts exhaust flow and backpressure, and increases the pumping losses in the engine. The bearing system reduces the friction and increases efficiency, which is why ball bearing systems are so sought after.
The 4LHR utilized a cast-iron bearing housing while the T-46 design used an aluminum bearing housing. Note the cutaway photo where the oil hole drilled for the bearing’s oil supply also has a much smaller hole extending up through the bearing housing where oil is sprayed onto the inside of that portion of the bearing housing exposed to turbine heat for cooling purposes. The cast-iron bearing housing offered better long-term durability and helped isolate the bearing system from heat.
The turbine of each model contains some other notable design feature differences. The Cummins T-46 uses an open turbine housing whereas the 4LHR uses a divided turbine housing. On a gas stand where turbine and compressors are run to map their performance characteristics, the T-46’s open type housing might possibly show as high or higher efficiency than the 4LHR’s divided housing. However, turbos run on engines and the exhaust gas pulses created by each exhaust stroke contain more energy than the period of time between each pulse. Pulse gas energy utilization is an extremely important design aspect that is worth discussing.
Since diesels run at comparatively lower engine RPM than gas engines to achieve the same power output, the amount of BMEP, is higher. This engine operating characteristic of diesels means that exhaust gas pulses are much more dramatic than are seen in a typical gasoline engine and are an important part of tuning a high efficiency turbine and exhaust system. For this reason, most diesel engines use turbochargers with turbine housings that employ the use of divided turbine housings to help transmit the exhaust pulse energy to the tips of the turbine wheel. In some cases, even the castiron exhaust manifold will resemble a tubing header design to keep the pulse energy isolated further allowing more pulse energy to reach the turbine wheel for more efficient on engine performance.
Exhaust gas pulse energy utilization is considered in the design of other engine components for both gas and diesel engines, it’s just more pronounced in diesels. You can find pulse energy utilization influencing the design of exhaust manifolds, tubular headers and their collectors for even naturally aspirated engines, and in the size selection of the primary tubes that lead from each exhaust port.
During the ’80s, new Cummins engines were equipped with a pulse manifold. This caused another aftermarket awareness that spawned retrofits of pulse manifolds to replace the older log-type manifolds where exhaust ports dumped into a collector and the pulse gas energy was greatly diluted.
The divider wall in the 4LHR turbo shows extending from the turbine- housing wall down to the tip of the turbine wheel inducer. The divider wall runs around the entire inside diameter of the turbine-housing scroll. This design feature is common to many turbocharger families used on diesel engines and is properly called a meridional divider wall.
The divided turbine housing was perhaps the most significant design component responsible for selling the 4LHR in the service aftermarket to replace Cummins brand VT-50, ST-50, and T-46 model turbochargers. It was also most popular with owner-operators running higherhorsepower engines than it was larger truck fleets. Truck owners could feel the difference in the 4LHR’s “early match,” as it was referred to. This meant that the turbo came on boost much quicker and the driver could feel it in the seat-of-the-pants dyno.
It should be duly noted that the Cummins turbo models discussed here were an amazing design due to their manufacturing cost efficiency and actually performed very well. In fact, as we discuss the design differences it is also fair to mention that if these two turbos were run on an engine at WOT (wide open throttlefull load) the Cummins original turbo design would operate as well and in some cases even better at max rated speed. This is again because of the divided versus open turbine housing design. At high engine RPM, the pulse energy tends to become more steady stream flow, and the exhaust gas pulses less significant. The real lesson to understand in this instance is that there are always performance trade-offs in design differences. At higher engine speeds the increased surface area inside of the 4LHR’s turbine created more total drag on the mass flow through the turbine than did the open housing causing a slightly less efficient turbine. The 4LHR allowed drivers to feel the difference in the low-end and midrange torque developed by the “tighter” turbine and divided housing design, but at rated speed, the open housing typically performed better.
When formulating your own system if you are applying it to a diesel, the application of divided housing is wise if your intended use is for low or mid-range operation. However, if the intended application is for high-speed/high power operation, like a Bonneville racer or tractor pulling, an open housing may end up providing superior performance. But if your turbo system is headed for a gasoline application trying to use pulse energy by use of a divided turbine housing will not likely be something that will deliver measurable benefits.
In the early ’70s Cummins purchased Holset Engineering located in Huddersfield, England. Holset, a former licensee of Schwitzer, designed turbochargers for the European market. By the mid-’80s Holset turbochargers were used extensively on Cummins engines and their design of the then new HT3B had notable similarities to the Schwitzer 4LHR upgrade. Cummins used the Holset turbo technology to upgrade their own turbos in the replacement market. Today Holset is known as Cummins Turbo Technologies, and is a world leader is advanced turbocharger design and technical development. Their patented design for a variable geometry turbine uses a slider ring claimed to be more reliable than the use of a variable nozzle ring design used by most others. That Holset design is now applied to the new 6.7-liter Cummins engine in Dodge trucks.
Making Modern Diesels Perform
Gale Banks Engineering can be credited with taking the major role in the development of the high-performance light-duty diesel aftermarket. In 1982, Gale Banks was pulling his Bonneville racer to the Salt Flats for the annual event known as Speed Week. He had just gotten a new Chevrolet diesel pickup truck powered with a 6.2-liter diesel. Being naturally aspirated and towing up mountains, the problems were many. Upon his return the “Duke of Duggan” (the company is located on Duggan Avenue), applied his knowledge of turbos to the Chevy pickup and the turbocharger retrofit kit for Ford and GM diesels was born!
Not to be outdone, Dodge teamed with Cummins in 1988 and released their own diesel-powered truck using the famous B-series engine, which was a 5.9-liter turbocharged, direct injected engine. Diesel enthusiasts flocked to show rooms and bought these vehicles because now a consumer could own a Cummins engine and park it in their garage. Thus the traditional horsepower and engine wars, so famous between the Detroit-based competitors, was reborn in the diesel truck market. Banks still sells the retrofit kits for the early model 6.2-liter GM 6.9 and 7.3-liter Ford diesels, but both Ford and GM have long since released factory turbocharged enines to compete with the Dodge-Cummins. As Gale Banks often says, “a diesel just isn’t finished until it’s turbocharged.” As we’ve mentioned diesel fuel has a higher BTU content (more potential thermal energy) than gasoline. A typical diesel engine cannot breathe on its own with enough volumetric efficiency to intake sufficient air for complete combustion. For this reason, modern diesels cannot even pass emissions without a turbocharger and an aftercooler to increase the density of the air charge.
With today’s electronic fuel systems, diesels have become relatively easy to hot rod. Instead of modifying your muscle car with cams, pistons, headers, intake manifolds, and more, you can add 100 hp to diesel using one of the hand-held programmers that sell for $350 to $750 and take just 10 minutes to install! But you can’t forget the fact that an engine is first an air pump. A diesel will continue to make more power as fuel flow is increased, even when there is too much fuel. Black smoke is clear evidence that unburned fuel is escaping through the exhaust. But unlike a gasoline engine, when a diesel runs rich, EGT (exhaust gas temperature) climbs very quickly, and internal engine damage is possible. So you can only “just add more fuel” to a certain point.
Today the design and manufacturing of high-performance diesel upgrades is a multi-million dollar market. High-flow intake and exhaust systems to increase total airflow abound. But regardless of how good these systems are, available airflow (and thus the amount of fuel you can burn safely) is ultimately limited by the stock turbocharger. While the early light-duty diesels could be greatly enhanced by the addition of an aftermarket turbocharger kit, all of today’s diesels are very high-tech and come from the factory with good turbochargers. However, as in the past, there are upgrade choices for today’s late-model diesels. Hopping up today’s diesel is all about a larger turbo with more mass-flow capability, or making modifications to the existing stock turbo and/or wastegate setup.
Honeywell Turbo Technologies, manufacturer of Garrett brand turbos, recently released a line of upgrade turbochargers called PowerMax. PowerMax turbos produce higher airflow and higher efficiency for cooler and denser intake airflow. One of the challenges to applying a larger turbo for more airflow to a street driven diesel is increased lag caused by the larger moment of inertia and the greater power requirement to drive the larger turbine and compressor. For this reason, the PowerMax turbos include Garrett’s patented true ball-bearing system, which lowers the drag on the turbine shaft and allows 15 percent faster spool-up. This tends to address much of the turbo lag complaints found from retrofitting a larger high-flow turbo that uses larger diameter wheels. The ballbearing concept, also available in other aftermarket turbos, is much more expensive to manufacture than the more traditional journal type bearings, but it’s benefits can actually be well worth the extra expense.
The PowerMax turbos are, for the most part, drop-in replacements for the stock turbochargers that come on the Ford, GM, or Dodge diesel pickup trucks. However they are all still called kits because there are various types of adapters such as air intake step-up adapters to accommodate the larger inducers on the turbocharger’s compressor cover.
The following charts show the currently available PowerMax turbos and the year, make, model of diesel truck they fit.
Note that each turbo references the application of an oil restrictor. It’s quite common for traditionalists familiar with engine building, to tend to want to open up oil passages for increased oil flow to maximize cooling and lubrication capabilities. But in the case of a ball-bearing turbocharger, the oil restrictor is a highperformance device itself. The oil passage in the bearing housing is drilled for a sufficient supply of enough oil volume for the hydrodynamic bearing system where journal bearings are used. Ball bearings do not require the same amount of oil supply. If you add in the extra oil, much of the efficiency improvements derived from using ball bearings would be negated by the bearings rotating through the viscous oil. So, don’t outsmart your ball bearing system—leave the oil restrictor in place!
There are also other high-performance options available for the diesel enthusiast. Banks, BD Power, and Holset are among the many other choices for turbocharger upgrades.
When deciding to hot rod your diesel-powered truck, car, or SUV, be sure to include a pyrometer to constantly measure EGT. Most pyrometers are reading T4, which is turbine outlet temperature. Each manufacturer will state what the maximum safe exhaust temperature is and this value is typically expressed as the maximum temperature the valves will sustain without permanent damage. Usually the temperature they refer to is T3, or turbine inlet temperature. When reading EGT from a probe placed in the turbine outlet (T4), be sure to account for the 200 to 300 degree F drop seen between T3 and T4. A safe bet would be to add 300 degrees F to your T4 pyrometer reading to get a safe estimate of your T3 temperature.
Tractor pulling is the original start of performance diesel application engineering. It dates back as far as 1929 when the first recorded pulls took place in Bowling Green, Missouri.
In his Monograph on Turbocharging published in 1953, Alfred Buechi discusses power increases from a diesel engine as a function of the charging pressure ratio. He states that the output of a turbocharged engine, if scavenged and intercooled properly, continued to increase at pressure ratios as high as 4:1 without any increase in exhaust temperature. Hugh MacInnes extrapolated the data and stated that the output could continue to increase at pressure ratios as high as 10:1. Buechi experimented with intake-manifold pressure of 71 psig in 1909. He may likely have run even higher if he had the materials that would have withstood the stress.
Today tractor pull manifold pressures have far exceeded what Hugh MacInnes extrapolated was feasible. Advancements in materials, block strengthening, and methods of cylinder sealing have allowed incredible pressure ratios well above 10:1. However, you cannot achieve such pressures with a single stage turbo system. Two- and three-stage turbos are used with intercoolers between each stage to keep the air temperatures from skyrocketing out of sight. In many applications, water injection is used to cool the charge as well.
In most high-performance automotive applications, increasing fuel is virtually incidental relative to the increase in air mass flow. The boost pressures and resultant air mass created by these extreme diesel applications presents challenges to figuring how to physically deliver enough fuel. This is not a typical problem in virtually any other form of motorsports competition. Remember, air doesn’t make horsepower, burning fuel does. So it doesn’t do any good to flow more air than the fuel supply can use. Advanced tractor pull engineers have to look at both high pressures and high injection volumes per injection event. The horsepower required to accomplish this mission is staggering.
Columbus Diesel Supply in Columbus, Ohio, has been specializing in tractor pulling for nearly 30 years and in 2000, was named manufacturer of the year by The Puller Magazine. Their specialized work in the development of super high-flow turbochargers and fuel pump combinations has earned them an international reputation. European tractor pulling has become very popular and the team at Columbus is frequently working with various U.S. and European- based teams. In addition to actually making highly specialized fuel injection pumps machined from billet aluminum, Columbus Diesel can actually run these pumps at track engine speeds—a unique capability. This is no small task. Most fuel injection test benches are made with a 20-hp or less motor to run calibrations on a variety of different make and model of fuel injection pumps. Columbus Diesel uses a specially modified test stand that has a 100-hp motor just to drive the fuel pump! Even with this level of horsepower on the test stand, their highest flowing pumps will bring the specially designed test stand to its knees. Tractor pull applications are in a category all by themselves. This is the very definition of extremists in action. Most OEM engineers shudder at the application variables seen in pulling tractors, like engine RPM that more than double the original max governed speed and boost pressures of over 200 psi!
In order to match the unique requirements of tractor pulling, Columbus Diesel Supply also manufactures unique combinations of turbochargers with flow characteristics not found on commercially available turbos. There are no turbochargers readily available for many of the engines competing in particular professional classes of tractor pulling. They also have been employing the use of special ball bearings for over 10 years.
The advances in fuel delivery and turbochargers for the sport has placed more emphasis on engine building and the ways to make these engines live under extreme stresses for just the length of the pulling track, which is only about 300 feet.
Written by Jay K. Miller and Posted with Permission of CarTechBooks