March 2015 Top Diesel Tech Questions
You’ve got questions? We’ve got answers!
Welcome to Top Tech Questions. One of our favorite forms of reader communication is tech questions. Our Top Tech section is a place where you ask what’s on your mind, and we answer. Send us an email at email@example.com and ask away!
Question: Why aren’t diesels supercharged from the factory? I understand turbos on racing engines that need lots of boost pressure, but it seems like on some of the newer OEM trucks a supercharger would work quite well. Is there a reason we haven’t seen one yet?
Answer: In the past, we’ve argued that superchargers won’t work well on diesels because of their high parasitic losses (basically, the amount of horsepower that’s required for driving a blower) compared to a turbocharger. Since blower dynos are hard to come by, and turbocharger dynos are almost impossible to find, we had to dig around to find an adequate answer for this one. In our own experiments, we found a turbo might use 30 to 50 hp, versus a diesel-sized blower, which could take as much as 200 hp to drive.
We did find some information that suggests there’s light at the end of the tunnel for diesel supercharging, however. We ran across some blower-dyno data on a 2.8L Kenne Bell supercharger that indicated it only took 111 hp to drive at 20 psi—not that much different than a diesel’s turbocharger. With newer stock diesels it’s not inconceivable that a supercharger could be used, provided the engine is revised with an improved camshaft and better flowing cylinder heads that would keep the boost down. Superchargers have a much narrower power efficiency range. That same Kenne Bell took 223 hp to drive at 26 psi. Perhaps the most interesting data confirming a supercharger could work on a diesel is a dyno graph of a supercharged Shelby Super Snake gasoline engine that shows 700 lb-ft of torque at just 3,000 rpm, which is diesel territory. This confirms the supercharger can provide enough air down low to make the large torque numbers we’re seeing in diesels.
Also, Audi has announced it will be selling diesels with 48-volt electric superchargers, or e-turbos, in the near future.
When it’s all said and done, engineering costs, the cost of the supercharger itself, and the fact that it is a slightly less efficient option, is the reason why OEMs don’t pursue supercharging diesel vehicles.
Question: I just ordered my first diesel—a ’15 Ford F-250 with the 6.7L engine—and I have a few questions about the DPF regeneration cycles. I normally only drive the truck about 7 miles a day at low speeds on the way to work, and it doesn’t seem to be going through a regen or, if it does, I don’t notice it. I’ve also heard of passive and active regens, but I am unsure of the difference. If I only drive these short distances, will the truck still go into regen, or not? I ordered it with an option to do a parked regeneration, but I’m also unsure of how often these need to be done. I’ll gladly pay for an honest answer, as I can’t seem to get one about regens!
Answer: We can definitely give you some advice on the matter, as it can be a little confusing. Passive regeneration occurs whenever the exhaust gas temperature is greater than 575 degrees, which is the temperature the DPF is able to start becoming an efficient soot cleaning device. In other words, unless your truck is at or near idle, there is some sort of passive regeneration occurring at all times during normal driving. The good news about passive regeneration is that your engine doesn’t use any more fuel than it normally would burn to heat up the DPF, so passive regeneration shouldn’t affect fuel economy. Active regeneration, on the other hand, is a different matter. Sometimes normal driving isn’t enough to clean out the DPF completely, and an ash (soot) buildup can start to occur. When this happens, the engine goes through an active regeneration process, using late injection timing and additional fuel to create exhaust gas temperatures in the 1,100-degree range to completely clean the DPF. This process takes somewhere between 9 and 20 minutes to complete and is the type of regeneration process most owners complain about when their fuel mileage drops.
There is some good news, however, as these regeneration events no longer need to occur at highway speeds. As technology has progressed, DPFs in the newer 6.7L Fords are now able to achieve active regeneration at speeds as low as 30 mph, which means even in city driving, they’re able to burn off excess soot that’s accumulated.
Active regeneration can happen as often as a couple of times per tank, or only once every few tanks, but we’re guessing you’re not going to notice when it does on your short drive to work.
One thing Ford is very specific about is not continually ignoring the message center when it says: “Drive to clean exhaust filter.” If you get this message, drive the truck, and it keeps popping up, that’s when we would recommend performing a parked regeneration cycle or, as Ford calls it, an Operator Commanded Regeneration (OCR). During the OCR, both engine rpm and exhaust gas temperature are greatly increased, and the engine is run until the filter has burned off virtually every last particle. If you do ignore this message, that’s when the DPF becomes in danger of being clogged and must be cleaned out manually.
Unfortunately, there’s no mileage or time interval we can give you for an OCR, as each person’s driving habits are so different. Some trucks that always tow heavy and are constantly at higher exhaust gas temperatures may not need an OCR in years, while others that idle often or sit in traffic could need one after a few thousand miles. The truck’s computer has a closed-loop temperature sensor that will throw a code if the truck is unable to fully clean the DPF, so its important part is to pay attention to the message center, or any fault codes that pop up, as this could be an indication that an OCR is in order.
Exhaust Manifold = Mileage?
Question: Just wondering if there is anything I can do with the exhaust manifold that would help mileage on my ’98 Cummins 5.9L. I’m looking for mpg and towing ability. Any recommendations on this would be great.
Answer: As odd as it sounds, an exhaust manifold upgrade can indeed help fuel economy, but the gains might not be as much as you expect. The secret lies in increased flow and reduced backpressure as compared to the factory versions. Especially when a diesel truck is turned up, the higher rpm levels combined with the higher volume of fuel can make the stock manifold become a hindrance, as the excess backpressure will lead to more horsepower when going down the road. And the more horsepower it takes to move a truck or turn the engine, the more fuel it’s going to use.
The downside is that the gains aren’t that huge. With the more popular big rig exhaust manifolds produced by companies like Bully Dog, a 0.3- to 0.4-mpg gain is advertised. On a truck-sized diesel such as the Cummins in your ’98 Dodge, the gains might be even larger, but you’d still be looking at putting on a lot of mileage to match the purchase price. In a semitruck that travels 100,000 miles a year, 0.3 mpg can add up rather quickly; in a truck that tows a camper 5,000 miles a year, it will take a lot longer.
But, it’s not all bad. In addition to providing a slight bump in fuel economy, aftermarket manifolds are stronger, available in stainless steel and/or with a powdercoated finish (or a variety of other options), and less prone to cracking than the factory versions. If you plan on upgrading your power levels or turbocharger down the road, replacing the factory manifolds now can save a lot of future headaches.