Gas vs. Diesel: Which Makes Most Sense?

29
2080
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It used to be true that diesel engines were simpler, cheaper to maintain and much more fuel-efficient as well as longer-lived than gas engines.diesel vs. gas image

It’s not as true today.

Current diesel engines are very complex – especially their emissions control systems. They have to be, in order to pass muster with Uncle – who would otherwise forbid their sale. They have particulate filters and urea injection, the latter requiring regular top-offs with diesel emissions fluid (DEF).

Diesel fuel also costs more than gasoline. Sometimes (as in my state) it costs more than premium unleaded gasoline.

It used to be the reverse. Diesel – once upon a time – cost less than regular unleaded. But that’s no longer the case because of the additional refining required to make diesel into ultra-low sulfur diesel – in order to be legal to use in modern diesel-powered vehicles (including heavy trucks).

Diesel engines still have a mileage advantage – but it’s not as great as it once was, in part because the emissions controls (and ultra low sulfur fuel) have reduced the fuel-efficiency of diesel engines (ask a big rig driver).

Also, gas engines have closed the gap.

New cars capable of 40 MPG or more on the highway are fairly common now. A 2016 Mazda3 sedan, for instance, rates 41 on the highway. This is only 5 MPG off the pace of the TDI (diesel) powered 2015 VW Jetta. And – as of this writing – there is no 2016 TDI-powered Jetta (or any other TDI-powered VW model) because of a federal fatwa forbidding VW to sell them… because (say the Feds) the TDI VWs are not emissions compliant.engine image 1

It will take more complexity (and so, cost) to make them compliant – and their mileage numbers will suffer on account of this, too.

Longevity used to be another diesel advantage – but that advantage is less today, too. For one, modern gas engines last a very long time – 200,000 miles or more – before they get tired. A diesel engine may last longer, but how many people keep a car longer than 200,000 miles? When gas engines routinely wore out by 100,000 miles (if not sooner) a diesel that could be depended on to go twice that (and which also delivered twice the mileage) had a lot of appeal.

That appeal isn’t what it used to be.

Diesels also no longer have the Big Torque/ Right Now advantage they used to have over gas engines. Because – like diesels – more and more gas engines are turbocharged.

These are new-design (twin-scroll) turbos, too – designed to build boost (and so, torque) almost immediately, replicating diesel power output at low engine speeds. For example, the 2016 Lexus NX200t I recently test drove (review here) has a 2.0 liter gas engine – turbocharged – which produces 258 ft.-lbs. of torque at 1,650 RPM. A rival – the 2016 Mercedes GLC300 – also has a turbocharged 2.0 liter gas engine and it makes 273 ft.-lbs. at an even lower 1,300 RPM.gas graphic

Compare that with the dearly departed TDI Jetta’s engine, which is also the same size (2.0 liters) but only makes 236 ft.-lbs. of torque at 1,750 RPM. The VW’s diesel engine makes a lot less horsepower, too: Just 150 vs. 235 for the Lexus’ 2.0 liter turbocharged gas engine and 241 for the Mercedes’ 2.0 gas turbo engine.

The combination of diesel-like torque output at low RPM and high horsepower give excellent off-the-line grunt as well as pedal-to-the-metal acceleration.

Plus pretty good gas mileage.

And you’ll spend less on gas. About 50 cents per gallon less.

This adds up.'76 Datsun ad

Bottom line, it’s getting harder to make an economic case or a functional case for a diesel engine over a gas engine, at least in passenger cars.*

Federal fatwas have chipped away at every advantage diesels once had over gas engines, by making them complicated and expensive both to buy and to maintain, as well as not particularly fuel-efficient… just like modern gas engines. Which – despite all their technical advantages – are less fuel-efficient than similarly sized engines were 40 years ago. A 1976 Datsun B-210 (with a 2.3 liter gas engine) was rated by the EPA as being capable of 41 MPG on the highway… exactly the same highway mileage posted by the 2016 Mazda3.

But a same-era VW Rabbit diesel was capable of 50 MPG – and it cost less to fuel because (back then) diesel fuel cost less than regular unleaded gasoline. VW also touted the diesel engine’s lower operating/maintenance costs vs. a gas engine, which was a function of the diesel engine’s simpler design and also the longevity-enhancing lubricating qualities of the diesel fuel available at the time.

The Feds have made it impossible to sell a car like the Rabbit diesel, so instead we get cars like the TDI Jetta.Rabbit

Well, we used to get them.

Now we can’t have them anymore, either.

VW was the only company selling affordable/efficient diesel-powered passenger cars in this country. It is not selling them now – and may never sell them again. Chiefly because it makes less sense for VW to make them – because it makes less sense for us to buy them.

*Caveat: Diesel engines still make sense in trucks used to pull and haul things, not so much because of their mileage (or even torque) advantage but chiefly because of the stouter construction of the diesel engine (heavier, usually cast iron rather than aluminum block, etc.) which has to be built that way to endure a compression-ignition engine’s very high cylinder pressures. Diesel engines – and the transmissions used behind them – are heavy-duty and can stand up to hard work better than gas engines.

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29 COMMENTS

  1. Diesel retains one huge advantage over gas.

    It can be stored much longer before going bad.

    If you cannot conceive of any scenarios where that could be priceless……..well then, never mind. 😉

  2. Advantages of desiel over gas:

    Diesel vapor does not catch fire as readily as gasoline.

    If diesel replaced gas refiners would not have to produce 2 fuels.

    I don’t think diesel uses ethanol additives.

      • George, diesel doesn’t have ethanol but it has, for the most, friggin bio-diesel mixed in. I was hauling a bit over 600 miles every day, dead-heading back so it was the perfect time to check mileage. I had always stopped at theT/A because it was easy and set up the way I liked and had large shade over the pumps and kept the rig and you in the shade, a big deal in west Tx. But they put a new Alon in town, not as easy to access and with a PITA payment method that I noticed had straight diesel. The Love’s across the interstate from the T/A which was 10% biodiesel had 15% biodiesel.

        Since I run as close to 40 tons every load there’s not variation. I wrote down the mileage every day between those three and voila, the Alon with straight diesel beat the other two and the T/A with less than the Love’s didn’t degrade engine performance as the fuel in the truck got hotter as the day went on. I also noted how a truck would pick up power if you got really low on fuel and had to fuel up before the end of the day.

        Maybe biodiesel expands more as it heats. I don’t know. But the straight diesel got the best mileage and fueling up mid-afternoon gained a few HP over hot fuel. I don’t explain it, just report it.

  3. The current emission fitments attached to diesels are soon to be applied to gasoline engines. Just wait, it will get much worse just so everyone can be shoehorned into electric buggies.

    Best of luck attaining highway mpgs, or even meeting the combined estimate with gasoline. That deep overdrive transmission will constantly downshift so that 1.2L turbocharged power plant screams out enough horses to keep up with traffic. Tiny engines force fed to produce voluptuous torque and horsepower will also consume sufficient fuel to do so, physics demands it. But EPA and CAFE tests can be manipulated by manufacturers to achieve what is unobtainable in the real world. So only drive downhill, with a tailwind, at 80*F, and no accessories engaged.

    I have seen those new fuel sippers on long and winding mountain inclines, seemingly able to maintain the speed of surrounding traffic but unable to accelerate around slower vehicles once that momentum is lost. The lack is especially apparent when the weight capacity is obviously maxed out, like when the whole porking family and their overstuffed luggage goes on a road trip. No 41 mpgs happening.

    • You’re right, CC.

      That Lexus NX I just reviewed? It averaged 26.4 MPG.

      Big whoop.

      That’s with both direct and port fuel injection, turbocharging and intercooling.

      All in all, about 10 MPG better than what a typical carbureted V8/RWD sedan delivered circa 1974.

      Which did not cost $40k.

      It’d be funny if it weren’t so got-damned sad.

  4. What you need is an old-school diesel from before the emissions fatwahs made them so complex and hard to service.

    With a few exceptions (like the catastrophic Olds diesel) they’ll run forever, get great mileage, and if so inclined you can convert them to run on waste vegetable oil.

    • I did some digging on those old GM diesels.Contrary to what is said, they were built ground up to be diesels. One of the main(main bearing)failures was caused by people not using CD diesel oil and using gas engine oil. Another problem back then(and this I remember well)was water in the fuel and no water separator on them. That was the cause of bent crank rods and cracked heads and broken crankshafts. The blocks and heads are highly desirable by racers to turn into gas engines. The owners tended to be, from the ones I knew. clueless non-maintenance people which sped up their demise. The heads didn’t have enough meat to be decked and hold up to diesel service which happened to a lot of them, a shop rebuild due to above problems.

      • The biggest issue of all with the Olds 350 Diesel was that it was put in Delta 88’s with automatics and driven by clueless clovers. Lugging and overspeeding both wreak havoc on engines. I never had a problem with mine, but I know how to drive a diesel.

        • You probably serviced yours too. I didn’t know anybody with one that did their own service so they took it to some clueless dipstick at a station and most likely got gasoline oil instead of diesel. But the amount of water in fuel was a big problem back then and it wasn’t from the refinery but from the distributor or retailer. A lot of people shoved the water hose after hours into their underground tanks and made some big money selling water. It was the same with gasoline too.

          I doubt those Delta 88’s were lugging the engine since it was an automatic transmission only. They might have over-revved them but that’s not too likely either. No water separator, a sorry fuel filter and clueless service probably were the main sources of breaking. Very few people had a clue that diesel needed an oil that was CD so they got the oil the guy at the station put in everything. Stick some 10W-30 in that Detroit or Cat and watch it come apart. I sometimes take a small break middle of afternoon and check the oil. That 15W-40 Rotella T comes off that stick like alcohol. I’ve left diesels sitting for 8 hrs or more and they were still hot to the touch.

      • @8SM,

        I owned one of the Oldsmobubble Cutlass 350 conversion diesels. GM eventually replaced all the conversion engines with target true diesel engines whenever one came into the stealership. That is if you didn’t have third party extended warranty coverage. The problem my engine had was breaking head bolts. Had the head bolts replaced twice on the engine for 100 bucks each time. The deductible you know. After the extended expired. I junked the crap conversion diesel and put in a 400 Oldsmobubble gas engine. That car ran great until a punk stole it and totaled it.

  5. Maintenance means different things for different shops. Change oil and filter, check and rarely change air filters or clean, oil and replace, check fluid levels and not much left. It’s good to change fuel filters on some sort of mileage check and not date. You can lube the hood springs on those that have that in the book, put some non-grease lube on door hinges and latches but mostly the costs are oil and filter and if you take it to some place where they specialize in this, they’ll change the air filter if you take it in every week. I check the power steering fluid fairly often and only have to look at the brake reservoir to see if the level is good, same for coolant. If you simply look at your wheels and rotors these days you can almost always tell pad wear without taking a wheel off.

  6. Diesel here in Texas is on par with regular unleaded, so maybe it’s a Virginia tax.

    I don’t know if you’ve shopped for a used TDi recently, but the prices have really dropped. Two years ago I was shopping for an ’03 or earlier VW TDi. Prices for a non-ragged out, non-wagon, manual Jetta was about $5k, Golf’s were $6k-$7k, and Jetta wagons were nearing $10k if you could find one. Anything with under 200,000 miles commanded a premium. I decided the car wasn’t worth it at the time.

    The same cars today are several thousand less. I just picked up an ’03 Jetta TDi wagon for $3500 with 154,000 miles. I’ve seen decent Golfs and Jettas similarly discounted from just two years ago.

      • Eight,

        The wagon was southwest of Houston in Needville. Showed up on ebay. AC doesn’t work, interior needs a thorough cleaning, and I expect typical maintenance stuff (shocks, brakes, tires). I haven’t seen it in person yet, but ebay requires (and the owner confirmed) listing all known problems, which means it should make the 150 mile trip home to southwest of Austin – I’ll find out on Sunday. I figure as long as there’s no drivetrain problems, I’ll come out ahead.

        FYI, there’s an ’01 Golf TDi in Kerrville for sale on craigslist that I was going to check out that looks to be a good deal.

  7. Is it a bad idea to shop for a late model (say 2010 – 2012) VW TDI? Like a Passat or similar? Or should we gobble them up? It would be for long-term ownership.

    • All the late model TDIs, at least the 2.0 liter, 2009 and up, are under the EPA fatwa. Depending on your state, if you buy one you may not be able to register it. If you don’t care about that, keep in mind that soon there are expected to be very few left on the road, so they will stick out like a sore (name your favorite appendage).

      • But that same TDI engine will be an awesome option for a swapper. I’m thinking that it will be amazing in a 1300 lb VW kit car!

        • Great engines but a nightmare to transplant. The pre 1999 1.9idi is a much easier swap. Not as powerful but still a good engine.

          From Acme Adapters;
          ‘later 1999 and up TDI’s are more complicated to swap and require the engine computer, dash instrument cluster, ignition key––all matching from the same serial numbered vehicle. This is due to VW’s immobilization and anti-theft control system.’

            • BTW, there is a procedure to swap out the TDI pump which eliminates some of the issues but the 1.6/1.9 idi will last much longer than a TDI with far less maintenance. The TDI mass air flow unit seems to be a problem for a few people I know. The idi units are so simple that unless you overheat them, they are near indestructible in my experience. Just make sure to add enough lubrication to the fuel for the injection pump.

  8. What I’ve observed differs slightly from your commentary.

    1. Diesel prices are not universally higher than premium gasoline. There are several factors that enter into the price of diesel vs gasoline, amongst them, the formulation of gasoline season-to-season, the draw of home heating oil from the pool of overall available diesel, the state of the economy in relation to shipping of products, etc.
    Presently, I see several stations, here in NY, where diesel is cheaper than regular and those are at prevailing low prices relative to other stations for both types of fuel. We had the combination of a warm winter, a depression in product shipping, etc which led to diesel being cheaper overall, despite extra federal road tax disfavoring diesel.

    2. We live in a controlled and manipulated economy, and it is going to get worse before it gets better. What fuel is used to drive the food trucks? Would it be expedient to cause that fuel to be more expensive long-term than the recreational and personal work fuel in a controlled economy, where controllers want to force workers onto trains and into inner-ring suburbs.

    3. Having vehicles which can support different fuels provides a hedge against an imbalance in price of any one fuel. It’s a similar argument one could make for the legitimacy of electric vehicles, if only they didn’t need to be heavily subsidized to survive.

  9. All this is true, I’m sure, but I can tell you one thing I’ve seen a difference in.

    Regular maintenance cost. Take the ICE in for service vs the diesel. My experience has been that the diesel is cheaper…much cheaper. My old ICE would cost 400+ for a service interval vs the diesel of 250.

    • I don’t think that will continue with all of the emissions controls crap being piled into the diesels now. You describe a low-complexity scenario. Direct injection, SCR, and DPF are all contributing to complexity and parts to wear out.

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