When I was a teenager in the 1950s and early 60s, I could only find superchargers and turbochargers on the drag strips around Orange County, Calif. Our local airport – now John Wayne International – was the place for all of the legal drag racing in the county. (There was also a lot of less legal drag racing happening on any road that was straight enough for a quarter mile with some braking area at the end. But I digress.) The dragsters of that day were mostly stock car bodies with engines that had superchargers and turbochargers sticking out through the hood. It was an awesome sight to see one of those beasts go down the quarter mile in seconds.
Far from being a thing of the past, turbochargers are approaching a new heyday, according to a recent forecast from Honeywell, a leading global developer of automotive turbochargers. It not only sees global turbo adoption rising to 47 percent of all new vehicles by 2020, but also an increasing appetite for turbo technologies that enhance a vehicles overall powertrain system, reduce complexity and are tailored to local market needs.
Honeywell unveiled its annual analysis just ahead of the Frankfurt International Motor Show, where dozens of automakers displayed next-generation turbochargers in their diesel, gasoline and hybrid models from entry level to mid-range to luxury segments. The mid-September report predicted more than 200 million vehicles with turbo engines will be produced during the next five years. It also said that turbocharged vehicles share of the North American market, now just 23 percent, will climb to 39 percent by 2020.
Some of you are probably wondering what the difference is between a supercharger and a turbocharger. Simply stated, superchargers are belt-driven and run all the time. Turbochargers – the subject of this column – use exhaust gases to do the work.
So whats the big deal here? To understand turbos, you need a brief education on the combustion process. We all know the combustion cycle: intake, compression, power, exhaust. What defines how much power we get is how much fuel we burn. The perfect air-to-fuel mix is about 14 parts air to one part fuel. Thats known as the stoichiometric ratio. If we allow the piston travel to do all the intake work, we will get a volume of air with fuel injected to make up the mix. However, if we push more air into the mix, we can add more fuel – and get more combustion power. Thats what turbos do.
Turbochargers are elegantly simple devices. On their way to the tailpipe, the engines hot exhaust gases are directed into a turbine which spins at up to 280,000 rpm. This turbine is harnessed to a compressor designed to force more ambient air through the engine. The air gets sucked in through the compressor inlet and its pressure is boosted. The charge air is then cooled to increase its density, and sent on to the intake manifold. From there, it is either sent directly into the combustion chamber and mixed with fuel, or fuel is injected just prior to going into the combustion chamber.
Turbochargers have been used in heavy-duty diesel engines for some time. They provide additional air to each cylinder, allowing for greater power output. In fact, almost all mobile diesel powerplants rely on turbocharging to some extent.
Recently, gasoline engines have begun to see more turbocharging applications, especially as fuel economy requirements force automakers to adopt smaller-displacement engines that result in lower power output. However, turbocharging can boost power, which will result in better performance without major fuel economy losses. All told, says Honeywell, turbocharged diesels can be up to 40 percent more fuel efficient than their non-turbocharged peers, and gasoline cars can see a gain of 20 percent.
Some of the most recent gasoline-fueled engines are about 1 liter in displacement and have direct fuel injection into the combustion chamber. With a turbocharger, these engines can deliver a lot more horsepower without breaking the fuel economy bank. Also emerging are 3-cylinder engines for the worlds entry-level car buyers. With their higher loads, these rely heavily on turbocharging for performance, and Honeywell foresees yearly sales reaching 7 million units by 2020.
Theres a price to pay, though, and the expected increase in turbochargers makes it important for all of us to understand its impact on engine oils – which is not insignificant. The main issue with turbochargers involves the bearing that supports the compressor and turbine shaft.
This bearing is exposed to very high temperatures from the exhaust gases which drive the turbine side. That heat is managed fairly well while the turbo is operating and the vehicle is running. When the vehicle stops and the engine is shut off, though, there is still oil in the bearing. This remaining oil sees a lot of heat, so it tends to cook. When it cooks, deposits form in the bearing until the clearances are too small to allow for easy startup. This phenomenon is referred to as coking.
According to industry sources and technical literature, the primary causes of coking are:
1. High temperatures in the turbos bearing housing.
2. Using engine oil that is not capable of operating in high temperatures.
3. Not changing the engine oil frequently enough.
4. Using engine oil that has a wide multi-viscosity range; some believe the additives used to achieve this are the material that causes coking.
I can agree with the first three of these points, but Im not convinced about the fourth.
As I mentioned, the high temperatures in the turbo bearing housing are definitely a cause and are addressed with a water jacket around the bearing housing to keep it cool. If heat is the problem, get the heat out.
Theres no question that an engine oil that is not sufficiently formulated to deal with the coking problem will not stand a chance. As far back as the early 1990s, Chrysler introduced a procedure called the Thermo-oxidation Engine Oil Simulation Test, which it developed with Savant Inc. to measure the coking tendency of an oil. The TEOST looks at oxidation and deposit-forming tendencies, but it may be that the test responds more to oxidation inhibitors than to deposit control agents such as detergents. Variations of the TEOST have been included in engine oil categories from API SJ (ILSAC GF-2) to the present API SN (ILSAC GF-5), all aimed at predicting high-temperature deposit formation.
In addition to an additive system that controls high temperature deposits, the base oil used in the engine oil formulation is important. The most ardent turbo gurus recommend only synthetic oils. Theres some merit in that, since the chemistry of synthetics lends itself to more stable oils at higher temperatures. However, there are some very good conventional oils that do well too, with the right package of additive chemistries.
I sure cant disagree with the statement that changing oil regularly is a good safeguard, as well. Once an oil begins to pick up sludge and varnish precursors – bad actors in the oil that havent quite deposited out yet – it may decide to leave those pieces where it gets the hottest. Changing the oil regularly will keep fresh chemistry and clean oil in the system.
There are those in the service and repair industry who think multigrade oils are a source of lots of potential deposit materials, and say only single grades such as SAE 30 should be used. I could see that position several API categories back, but with the more stringent test protocols and improvements in chemistry, I think were probably okay with multigrades.
Another issue that bears watching is how very-low-viscosity oils will do in the turbo realm. It appears likely that any ultra-light grade such as SAE 0W-16 might have a fair slug of synthetic base stock in it, so perhaps there wont be a problem.
In the field, a best practice to ensure that turbos last is to cruise at low RPM where no boost pressure is created before shutting the engine down. Its just like a cool-down walk for a champion racehorse after the derby.
Theres no question that you drivers of vehicles with turbos should use a high-quality oil that is formulated to deal with turbos, and should keep a close eye on your engine oil dipstick. Regular changes are an important part of that plan. Be sure to check your owners manual for the proper viscosity grade and performance category.
Now go out and burn up some road!
Industry consultant Steve Swedberg has over 40 years experience in lubricants, most notably with Pennzoil and Chevron Oronite. He is a longtime member of the American Chemical Society and SAE International, where he was chairman of Technical Committee 1 on automotive engine oils. He can be reached at steveswedberg
@cox.net.