Tag: Engineering Explained

  • Why MPG is a dumb unit for fuel economy

    Miles per gallon—mpg—is the default measure of gas mileage in the United States, but it may not be the best one, argues “Engineering Explained” host Jason Fenske. Here’s why he’d rather see a different unit of measurement for fuel economy.

    Fenske doesn’t have a problem with the individual units—the mile and the gallon—but says something “weird” happens when you put them together. He explains that with a question: Is it better to double the fuel economy of nine cars from 50 mpg to 100 mpg, or double the fuel economy of just one car from 5 mpg to 10 mpg?

    When just looking at efficiency, increasing the fuel economy of the nine 50-mpg cars is the best way to go. But once you factor in distance driven, it’s the other way around.

    Gas pump

    Gas pump

    That’s because mpg is a measure of distance (miles) per volume (gallon). Fenske notes at this point that most other countries flip this around, measuring fuel economy in volume per distance. In Europe, for example, the default unit is liters per 100 kilometers. Measuring fuel economy in gallons per mile would be more straightforward, he says.

    U.S. Corporate Average Fuel Economy (CAFE) targets are already calculated in a manner that’s closer to gallons per 100 miles, Fenske notes. They’re then converted back into mpg to make the numbers easier for the general public to understand, a process called a “harmonic mean.” Window stickers even show fuel economy in gallons per 100 miles in smaller print under the mpg figure.

    Increasing the mpg of two cars by the same amount also doesn’t mean they save the same amount of fuel. A gain of 1 mpg will equate to more fuel savings in a car that started out getting 5 mpg than a car that started out getting 50 mpg. Watch the full video to see the math supporting this, and if you want more efficiency, check out this video explaining how big tire sidewalls are your friend.

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  • Should you use racing oil in your road car?

    Every part of a modern race car is optimized to extract maximum performance—including the fluids that go into it. Specialized racing oil is a must in the paddock, but is it a good idea to use that in your road car?

    “Engineering Explained” host Jason Fenske answers that question in this video, talking to engineers from Mobil1 (which also sponsored the video), engine builder Roush Yates Engines, and NASCAR team Stewart-Haas Racing.

    First off, Fenske emphasizes that racing engines don’t have much in common with road-car engines. Pushrod V-8s like the ones used in NASCAR disappeared from the average American car decades ago, and racing engines in general have a very different duty cycle. They’re flogged (a NASCAR V-8 can rev to 10,000 rpm and spend 90% to 95% of a race at full throttle), but also have fairly short maintenance intervals. NASCAR engines have their oil changed about every 500 miles and are only expected to last about 1,500 miles, Fenske noted.

    Ford Mustang NASCAR Xfinity Series race car

    Ford Mustang NASCAR Xfinity Series race car

    NASCAR oil is also fairly unrestricted (rules can vary by race series), giving teams more leeway to look for competitive advantages, Fenske said. Teams tend to use thinner-weight oils to reduce frictional losses and maximize power. That’s not great for longevity but, remember, the engine only needs to last 1,500 miles. Road-car engines, on the other hand, are expected to last hundreds of thousands of miles, so that kind of tradeoff isn’t possible.

    Racing oils and road-car oils do have some commonalities, though. They use a lot of the same additives, such as detergents, dispersants, anti-foaming agents, antioxidants, and rust inhibitors, Fenske noted.

    However, the difference is in the details. Oils used for road cars are designed to meet a long list of regulations and requirements set down by individual automakers. In NASCAR where, again, rules governing oil are fairly loose, teams can optimize an oil for one specific engine design. It’s really that process that makes racing oil work, and it’s just not practical for individual road cars. Unless you have your own laboratory and a lot of patience, at least.

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  • What’s the future of the internal-combustion engine?

    With stricter emission standards and the rise of electric powertrains, it might seem like the internal-combustion engine’s days are numbered. But Engineering Explained host Jason Fenske believes internal combustion will live on—thanks to new technologies.

    Fenske is pretty optimistic about the longevity of the internal-combustion engine, both because of the inherent energy-density advantage of gasoline over batteries, and because of efficiency-boosting technologies. In this video, he takes a closer look at some of those technologies.

    One option is homogeneous charge compression ignition (HCCI). An HCCI engine burns gasoline, but uses compression ignition—like a diesel engine—rather than a spark plug. In theory, that provides the efficiency of a diesel, without the soot and high levels of nitrogen-oxide (NOx) emissions. However, it requires much finer control of the intake temperature, as well as the timing of ignition.

    Ferrari 488 GT Modificata

    Ferrari 488 GT Modificata

    The next option is pre-mix charge compression ignition (PCCI). Fenske described it as a “middle ground” between diesel-engine compression ignition and HCCI, because it injects some fuel early to let it mix with air in the combustion chamber, and then injecting more fuel later. That provides more control over ignition timing than HCCI, but can also create pockets of unburned hydrocarbon byproducts, which is bad for emissions. PCCI engines also have a fairly narrow operating range, with high potential for knock under full throttle, Fenske said.

    Finally, we have reactivity-controlled compression ignition (RCCI). This uses two fuels: a low-reactivity fuel (like gasoline) that is port injected, and a high-reactivity fuel (like diesel) that is direct injected. “Reactivity” refers to a fuel’s tendency to ignite under compression. This method leads to big efficiency gains, but still with fairly high emissions, Fenske said. The complexity of using two fuels could also make it a non-starter commercially.

    These alternative internal-combustion engine designs may not be ready for prime time, but automakers are looking to squeeze every ounce of efficiency out of today’s gasoline engines using more mature technologies like direct injection. Fenske also covered another possible future internal-combustion technology—entry ignition—in another video, which is also worth checking out.

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