Apr 22, 2024 at 11:23 AM
  1. TDRComm Staff Member

    QUESTION: Why is the diesel engine more fuel efficient than a gasoline engine?

    Just the other day a customer was having a conversation with one of the Geno’s Garage technical staff. The topic was the better fuel economy of the diesel engine over the gasoline engine, and the subtle mechanical differences in engine architecture.


    The customer was correct in noting these differences:
    • Diesels are “heat engines”, they’re designed to be more efficient.
    • Diesels use turbochargers.
    • Diesels operate with a higher compression ratio.
    • Diesels have more structure to support the turbochargers and the higher compression ratio.
    • Diesels have a better (higher pressure) fuel injection system.
    • Diesel fuel has more BTU content than gasoline.
    • Diesel engines operate at lower RPM.
    • Diesel engines are almost always “under square” (longer stroke) more torque.
    However the customer “danced around” the easy to remember, easy to explain response.*(scroll down for response)

    Here is where we toss this question back to you: For a given number of cubic-inches (or liters of displacement), why is the diesel engine more fuel efficient than a gasoline engine? If you would like some supporting documentation for your response, we have a recommendation: The Dodge /Cummins Historical Perspective, pages 6-11.

    TDR members, the Dodge/Cummins Historical Perspective book is another benefit of your TDR membership – access to the publication is free with your membership – along with numerous others.

    Considering a TDR membership? Annual membership to the TDR costs $35 for four print magazines, the annual TDR wall calendar, TDR forum access, and the following downloadable publications:


    However, if you simply want the Dodge/Cummins Historical Perspective book, non-members can purchase a copy for $9.95. Click HERE to purchase the downloadable Dodge/Cummins Historical Perspective book.


    * Here it is (in the simplest of explanations). The answer to why the diesel engine is more fuel efficient than a gasoline engine.

    The Injection System
    “It’s in the injection system.”
    Rudolf Diesel designed the heat engine to use the injection of fuel at the last moment to ignite the compressed air. Understanding the heart of the diesel, the fuel pumps, is the key to answering the fuel efficiency question.

    The Gasoline Engine
    A gasoline engine is what engineers call “stochiometric.” Stochiometric describes the quantitative relationship between two or more substances, especially in processes involving physical or chemical change. With a gasoline engine there is a stochiometric equation of 14 parts of air to one part of fuel. Remember, always 14:1. (Okay, modern gasoline development has some engines at leaner 16:1 and gasoline direct injection is now widely being used.) Whether at idle or full throttle, the fuel and air are (typically) mixed outside the cylinders in a carburetor or injection manifold, and the mixture is introduced to the combustion chamber via the intake valve, 14:1, always.

    The Diesel Engine
    Fuel and air in the diesel design are not premixed outside the cylinder. Air is taken into the cylinder through the intake valve and compressed to make heat. Diesel fuel is injected near the top of the piston’s stroke in an amount or ratio corresponding to the load on the engine. At idle the air-to-fuel ratio can be as high as 85:1 or 100:1. At full load the diesel still boasts a miserly 25:1 or 30:1 ratio! It is in the injection system where we find the key to the diesel’s fuel mileage superiority.
    TDRComm , Apr 22, 2024


Discussion in 'Articles' started by TDRComm, Apr 22, 2024.

    1. bry1216
      It all about the horsepower and torques. Gasoline has more horsepower but less torque. Torque is what makes the transmission turn the wheels. Less torque less fuel efficiency because more fuel is used.
      Thus a small Turbo charged 1.9 liter diesel engine gets better fuel economy than similar sized gasoline engine with similar transmission and wheel diameter.
      Diesel85 likes this.
    2. MCanning
      Google says it is because of higher compression creating heat and Thermal effeciency
      Diesel85 likes this.
    3. Diesel85
      Yep, the high compression, thermal efficiency, leaner air-fuel-ratio during less work periods, etc, etc.

      Funny TDR posts this today... China is making more progress on diesel engines! --
      China unveils world’s 1st diesel engine with 53.09% thermal efficiency

      Glad to see more progress is still being made on diesels. Even if it's by China. Even they know that diesel is still the best.

      I will forever and ever cherish diesel power. :D

      I'll never go to gasoline or EV.
      Carolinadave55 likes this.
    4. bry1216
      My answer above was about "Fuel Economy" and how it is related to horsepower and torque.
      About Earth effects:
      Gasoline engines exhaust is mainly carbon monoxide (CO) which kills everything on the planet. It is from wood, coal and gasoline. CO is lighter in weight than CO2 so it will be higher in atmosphere.
      Diesel engine exhaust is mostly carbon dioxide (CO2) and nitrogen oxide (NO) with a very small carbon monoxide (CO). Everything green on the planet absorbs CO2 and releases oxygen (O), thus it is important to keep tree growing! Even humans breathe in O and exhale CO2.
      Diesel under stress such as racing off the start line, under heavy load or high rates of speed will increase NO and a "representative in" black smoke!
      Diesel85 likes this.
    5. TDRGuy
      Mr. Rudolph Diesel, Google-guy, China-guy and TDR -guy: All are saying thing, it is a "heat" engine design and the thermal efficiency comes from being able to vary the amount of fuel needed to make the hot air go "bang".

      Diesel85 likes this.
    6. Diesel85
      Low CO is the biggest reason I like diesels versus gassers. I always feel choked out when behind any gas vehicle even brand new vehicles. I'm sensitive to gas fumes versus diesel.

      With all this nifty clean air tech on diesels, I'm surprised that diesel isn't being pushed more. I can see a lot more companies profiting off of clean air diesel stuff versus EV.

      I used to hate emissioned up diesels until a few years ago. Now I see the light at the end of the tunnel -- it's a refinement on Mr. Diesel's already brilliant technology. I would take an emissioned up diesel versus an EV or a more polluting gasser.

      I prefer to drive a diesel because it's scientifically proven to help the environment, keep me and others safer, and it helps plenty of industries in the manufacture of the truck.
    7. Tuesdak
      Today they are not. History is just that and makes a good campfire story starting out with "Long Ago..."

      Unloaded a Hemi 3500 gets the same MPG as my 3500 HO Cummins. We are even doorhandles from a dig at a stoplight. Gasoline engine MPG has come way up making this an "Old Timer Old School" discussion that is loosing it's relevance. My buddy's Power Wagon 2500 Hemi is getting 18 MPG - same as my 18 HO Cummins pickup unloaded. Meanwhile Emissions has reduced the MPG of diesel engines including an oil discussion we hardly have of ULSD diesel changes that reduce the BTU's per gal of diesel.

      I would get 21 MPG on ULSD on my 2wd MT 2003 and 18 MPG on a 2008 DPF equipped Duramax and same on my 2018 Cummins: best of 18 MPG. Oh yeah now add DEF to the TCO and I use a surprising amount of it loaded. 20 gal DEF in 1600 miles towing (!)

      Further it's a incomplete discussion to focus on just MPG when the TCO is the real measure on say your bottom line. The cost of maintenance, Higher Fuel Cost Diesel Vs. Gasoline, and over $10,000 more upfront cost, repairs "What's in your wallet?!" diesels are catastrophic and expensive ... There is no "Economy" left in a diesel. Diesel fuel has been more expensive than 87 octane gasoline by as much a $1.00 a gal here in the past 3 years.

      Used to be... It is the BTU content of the fuel followed by the air-to-fuel ratio. Further under load Gasoline engines have to go full rich not only for power, but, to prevent the EPA mandated too close to the engine for faster warm up catalytic converters from melting down. Yes, gasoline engines have to dump fuel to "put the fire out" in the converters or they WILL melt down. Yes the converters stop working and start stinking when this happens.

      Loaded towing, you didn't limit your discussion to "Loaded", you may get better diesel MPG as a gasoline engine dumps fuel to save the cats. How often are you loaded also matters as one trip a year doesn't make a dent in the usual 15K per year of driving.

      So why do I own a diesel? It's a like a sports car where the engine isn't crippled by the extreme heat we have by spark knock and when towing I don't have to listen to a Hemi Scream pulling my 5er up the grades. No it wasn't for MPG as there are many better vehicle choices if I only cared about MPG.

      After the MPG my buddy gets with his 2022 (or 2023) Hemi there is no longer a reason to brag about diesel MPG. "Yeah, So? I get that with my Hemi..." :p

      When the HP wars end and they drop in a smaller diesel focused on MPG than we may be able to brag again. Several 1/2 ton small diesel's have been dropped from the market: what's that tell you?

      This is only correct for turbo diesel engines. NA, like gasoline engines usually are, compared to NA diesels Gasoline had more HP and more TQ for the same displacement.
      Last edited: Apr 29, 2024
      HPSimpson and Diesel85 like this.
    8. Diesel85
      The reason the 1/2 ton diesels dropped from the market is the manufacturers focused on what they thought was going to be the next best hit: EV.

      I heard from several sales people at local dealers that the EcoDiesel just made sense versus the EV niche they are trying to fill. More sales were generated.

      If it's true what I've been hearing that for once pump regular gas will be higher or the same price as #2 diesel this summer, then owning a diesel is even more awesome.

      GM has no plans on ending the mini-DMax as they are seeing sales numbers starting to really gain some momentum. Partly has to do with the EcoD being killed off... Rumor has it they are working on potentially doing away with that oil pump belt system and doing a completely maintenance free chain system.

      If Stellantis/RAM was smart right now, they should kill off the RAM Charger and either bring back the EcoD or some kind of diesel back into the half ton segment to compete with GM.

      Ford got reamed on their Lightning EV sales. I don't have the figures, but Ford sold a boat load more mini-Powerstroke diesel F-150 trucks supposedly according to my local Ford dealer. Only because it made sense and more folks want diesels versus EV or even a hybrid truck.

      More people want diesels than EVs. It's a fact.

      Doesn't bother me much anyways, because I'm having fun in my EcoD, but I want to move onto a bigger truck as I get older for camping and for something bigger/heavier like my daily driver work truck. Though, not everyone is like me who wants to go bigger. Usually as people get older, they want a smaller vehicle. I'm hoping my wish list for a dream 2026-2027 RAM HD Cummins will have the following:
      • Crewcab design like the 1500 DT trucks with the flat floor in the rear
      • ZF Powerline 8 Speed
      • Full-time 4x4 transfer case option
      • Even more diagnostic info on the EVIC/Center Stack Screen
      Diesel should be made available to all. Passenger car to class 8 big rigs... Plenty of folks like small diesels.
      Last edited: Apr 29, 2024
    9. TDRGuy
      To Tuesdak's comment on efficiency given the same displacement: Good points, gasoline engines with modern turbochargers (and some natural aspirated engines) have made great fuel economy gains. But, I'll stick with the Diesel as more miserly of the two choices.

      Total cost of ownership and miles per dollar spent on the fuel of choice: Yep it looks like the gasoline-fueled vehicle is today's winner.

      It is great to have choices.

      Last edited: Apr 30, 2024

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