Diesel engines have been around forever—certainly as long as their gasoline-fueled counterparts. And while we have naturally aspirated, supercharged and turbocharged gas engines, the turbocharger remains the overwhelming choice in the diesel marketplace, as it’s found on virtually all modern diesel engines. This of course begs the question… why?
A LITTLE HISTORY
Some of you may be old enough to remember the smoky and hopelessly underpowered naturally-aspirated diesel cars and trucks that were made in the 1980s. While the diesel powerplants allowed these vehicles to get very good fuel economy, that was it; that was the only thing these vehicles offered. With 0- to 60-mph times in some cases nearing half a minute, the public responded by shunning these diesel offerings, and compression ignition engines were so rarely chosen that many manufacturers dropped diesels from their lineups.
In the end, it was heavy-duty pickup trucks that saved the day in the late 1980s and early 1990s, with Ford, Dodge and GM all jumping on the turbocharged bandwagon. Horsepower and torque levels started out innocently enough, with most offerings in the 160-hp range, which isn’t overwhelming for 3-ton trucks. However, as time progressed, power climbed, with the 2001 Duramax hitting the market as a game changer at 300 hp. Fast forward to 2014, and 400 hp (or more) ratings are common, with torque levels at 800 lb.-ft. or more. As time and power has progressed, the turbocharger has been with diesels every step of the way. It’s so important to making diesels a viable engine option that “turbodiesel” is most commonly written as one word. But why is that? Well, let’s look into it.
DIESEL AND TURBOCHARGING, A PERFECT MATCH
A diesel engine is direct injection, meaning that the combustion process starts when fuel is injected into the engine. In a gasoline engine, fuel and air are mixed and a spark plug ignites the fuel, but in a diesel the fuel itself interacts with the air and ignites just based on pressure and heat. Hence the term “compression-ignition” engines. This enables a diesel engine to operate on a wide variety of air-fuel ratios. Where gas engines are most comfortable at somewhere between a 10:1 and 15:1 air fuel ratio, a diesel can run with a fuel to air mixture as rich as 6:1, or as lean as 100:1. There are diminishing returns in power much past 20:1, but regardless, diesels are about as air-fuel independent as engines come.
But, there are drawbacks. Since diesel fuel isn’t mixed with air before it enters the engine, it only has so much time to burn and create pressure to make power. As rpm increases, the engine’s piston travels up and down at a higher rate of speed, giving less and less time for the diesel fuel to burn and make power. If you’ve ever wondered why diesel engines are traditionally lower-rpm engines, injection timing is why.
So, engineers found a way to cheat this internal combustion engine puzzle: turbocharging. A turbocharger is an exhaust-driven compressor that forces more air into the engine. This way, the engine can accept more air without more rpm; just more air from the compressor, which is commonly referred to as “boost.” With the correct amount of fuel added, this meant that diesel engines could now compete with their gasoline counterparts as far as power went, with both torque and effecenciey advantages.
“Increasingly stringent emissions regulations, along with demands of better response and more horsepower would lead to a new generation of turbocharger design that would continue on to today’s trucks.”
EARLY TURBO BASICS
Every turbocharger (then, and now) is composed of a few basic parts: the center section of the turbocharger which features oil-cooled and lubricated bearings, and a common shaft that connects both the exhaust and compressor sides of the turbocharger. There’s also the compressor side featuring a compressor wheel and compressor housing, and the turbine side featuring the exhaust-driven side of the turbocharger (called a turbine wheel) and housing. In the late 1980s and early 1990s, turbocharged diesel followed the same basic formulas: boost levels under 20 psi, fixed geometry (more on that later), and large exhaust housings and turbine wheels, to keep exhaust pressure down. The turbochargers were also rather small (the Holset HC1 on the 1989 Dodge for instance, carried only a 50mm inducer) and power levels were mild to match the moderate boost levels. Turbocharger design remained patterned after this formula until 2000, when the winds of change were blowing. Increasingly stringent emissions regulations, along with demands of better response and more horsepower would lead to a new generation of turbocharger design that would continue on to today’s trucks.
The common-rail injected Duramax diesel debuted as a 2001 model, and featured a much larger turbocharger compared to previous models. These larger Garrett turbos enabled the Duramax to produce 300 hp at the flywheel (far and away the most of the big three at the time) and its large 6.6L displacement and sophisticated electronic controls allowed the engine to make the most of its compressor.
“Not to be left behind, both Dodge and Ford offered advanced turbochargers in an effort to make up for the Power Stroke and Cummins’ horsepower deficiency to the Duramax.”
Not to be left behind, both Dodge and Ford offered advanced turbochargers in an effort to make up for the Power Stroke and Cummins’ horsepower deficiency to the Duramax. Dodge installed a variable nozzle turbocharger on its Cummins engine, which could effectively change the exhaust housing-side A/R by a factor of three. This led to a turbocharger that was very quick to spool, but could be tailored to reduce exhaust backpressure for mileage, or under high-load, high-rpm situations.
GM released the 6.6L Duramax with a massive (for its time) GT37 turbocharger. This motor made a mean 300 hp.
Ford unveiled the 6.0L Power Stroke equipped with a never-seen-before GT37 AVNT turbo. This turbo was the first variable-vane turbo offered in a pick-up and was capable of providing boost at just about any rpm.
Dodge/Ram installed a variable nozzle Holset HE351VE on the 6.7L Cummins. By changing the aspect ratio this turbo works similar to the 6.0L providing varying boost levels at almost any rpm.
Ford stepped it up a notch with a set of twin turbos on their 6.4L Power Stroke. With a bit of tuning and a few other simple modifications this powerplant was capable of 800 hp of smooth just-off-idle power.
Ford tries again by utilizing a new “twin” type setup on the new 6.7 Power Stroke. This Garrett-designed charger had two back-to-back compressor wheels, with a single turbine wheel.
Ford also offered a variable exhaust-side turbocharger, but used movable vanes that directed exhaust flow toward the turbine wheel to slow or speed up the turbocharger on its Power Stroke engine. In 2005, GM also added variable technology to the Duramax’s Garrett turbo. As of now (2014) all of the big three are still sticking with variable turbo technology.
EFFECTIVE AND UNIQUE DESIGNS FROM THE MANUFACTURERS
While it may seem like Ford, Dodge and GM all followed the basic path, there have been some strays from the norm. From 2008 to 2010, the Power Stroke engine found in Ford’s trucks carried two turbos, in a compound (one blowing into the other) arrangement, leading to a high-torque engine that also offered great power potential. Thanks mostly in part to their turbo arrangements, these Fords quickly became popular with the “tuner” crowd, which were able to effectively double the engine’s power with fuel and timing table modifications. These extreme power increases wouldn’t have been possible without this turbo arrangement.
Other European manufacturers have also experimented with various different designs of turbo systems. The BMW 335d for instance, has a very small variable geometry turbo as its small compressor, and it’s active from about 1,500 rpm to 2,500 rpm. At that point, the engine’s exhaust is diverted completely past the smaller turbocharger, and a larger turbo takes over from 2,500 rpm to 4,000 rpm. This turbo setup (called a sequential system) results in an engine with an extremely wide power band, and a very fun to drive vehicle— perfect for a sports sedan.
“With additional fueling and tuning, most diesel engines are capable of nearly twice their factory horsepower ratings—even with the stock turbochargers.”
AFTERMARKET TURBOCHARGER PERFORMANCE, FROM MILD TO WILD
With additional fueling and tuning, most diesel engines are capable of nearly twice their factory horsepower ratings—even with the stock turbochargers. However, at some point, the stock turbocharger just won’t cut it, and that’s where many aftermarket manufacturers have stepped in. Whether you have a 7.3L Power Stroke or a 6.7L Cummins (and everything in between), a turbo system exists to help you make 500, 600, even 1,000 horsepower. With the choices in turbos being numerous (and at times confusing) one of the most commonly asked questions by diesel enthusiasts is “what turbo should I buy?”
It’s actually not as bad as it all seems. For instance, turbocharger selection in most part should be based upon the maximum horsepower the engine is built to produce. That’s it, no voodoo or witchcraft. Whether in a compound or single arrangement, turbo selection should be horsepower based. It’s been our experience that most people over-turbo looking to make a ton of power. We’ve seen an ATS Aurora 3000 for instance (57mm S300-based) make 450-500 rear-wheel horsepower on both Duramax and Cummins applications. The new forged-milled wheel 67.7mm BorgWarner turbos are tearing up dynos and drag strips across the country, with a 700-plus horsepower potential. If you’re truly looking to use an 80mm turbo to its full advantage, you’d better be looking at 900 to 1,000 horsepower.
As technology marches ahead, we’re going to see innovations in turbocharging, just like everything else. One interesting experiment was the turbocharger on the 2011 to 2014 Ford trucks with the 6.7L engine. This Garrett-designed charger had two back-to-back compressor wheels, with a single turbine wheel. The idea was to have twin-turbo like airflow with single-turbo spooling. It allowed the truck to come into power incredibly hard, and sold quite a few Fords. However, the turbo ran into drive pressure issues when power was raised or at high altitudes, so the 2015 models went to a traditional turbo (two wheels) that was just a bit larger. Still, with the OEM starting to embrace this type of experimentation, expect to see more unique setups involving compounding, sequential turbos, and possibly even supercharged/turbocharged combinations.
In the aftermarket, turbocharging has taken an interesting step backwards. With the ultimate strength of new turbos on the rise, and companies like Garrett, Precision Turbo, and BorgWarner designing turbos especially for diesels, the usage of single turbos is on the rise. With compressors costing less than $1,000 that will support 50-60 psi of boost, and the advantage of electronic controls on newer common-rail engines, single turbos are very popular in the 500 to 800 horsepower street truck market. Anything above that, and compounds still reign supreme, with the exception of some specially designed turbos for pulling classes. One thing is for sure however, both in the OEM and the aftermarket, look for new designs, trends and innovations, as the turbo market is forever changing.
MATCHING THE ENGINE TO THE TURBOCHARGER: DRIVABILITY CONCERNS
If you’re wondering why everyone doesn’t run around with some huge single turbocharger capable of huge horsepower numbers, there’s a reason for that. The engine and fuel system must also be designed around a desired horsepower range, and often large turbochargers capable of obscene power numbers aren’t all that drivable. While a factory pickup engine might make power from 1,500 to 3,000 rpm, a large single might not even spool much until 2,000 rpm, and might have a power range from 3,000 to 4,500 rpm. Since many people use their diesels for diverse reasons (like towing) having an engine that makes almost no power before 3,000 rpm can be a detriment. That’s where a second turbo comes in.
A good solution for a drivable vehicle that still can still make power is compound turbocharging. Present in factory form on the 2008 to 2010 Power Strokes, compound turbos offer the spooling ability of a small turbo, with the ultimate power potential of a larger turbo. At high-boost and high-rpm situations, the workloads of the turbochargers are also lessened, resulting in lower compressor outlet temperatures and increased efficiency. Also, at part throttle situations, the compounding effect of blowing one turbo into the other results in more boost with the same amount of fuel (as compared to a single turbo) and makes compounds perfect for towing situations. With a compound turbo setup, a broad power band of say, 2,000 to 4,000 rpm, or 1,500 to 3,000 rpm can be realized. DW