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Diesel vs Gasoline Engine

  1. Sep 17, 2007 #1
    I hear diesel engines are better than gasoline engines. Why is that?
     
  2. jcsd
  3. Sep 17, 2007 #2

    FredGarvin

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    Better at what?
     
  4. Sep 17, 2007 #3

    mgb_phys

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    They are more efficent in extracting more of the chemical energy of the fuel - ie more miles per gallon. They can also use lower grade fuels which need less refining and so should be cheaper (they aren't because of taxes).
    Diesel engines don't need ignition systems so can be simpler but must run at higher pressures and so need to be stronger, this makes them last longer but are more expensive to build.
    They emit more particulate pollution but less NOx - opinions differ as to wether they are overal better for the enviroment
     
  5. Sep 19, 2007 #4

    brewnog

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    Diesel: more efficient, higher torque for lower speeds, no inherent reliance on electricity. Heavier, and have the need for more expensive ignition systems. Very receptive to turbocharging.
     
  6. Sep 19, 2007 #5

    NateTG

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    Diesel engines do not have more expensive ignition systems, but they do have significantly more expensive fuel injection systems. Moreover, diesel engines are not intrinsically heavier than otto engines, but, rather, tend to run at higher compression (which is also why they work so well with turbochargers).

    Diesel engines are more efficient because they can run at higher compression, which works out to better thermodynamic characteristics. Otto cycle engines would have premature ignition (a.k.a. knocking) at those pressures.

    Historically diesel engines have been heavier in order to accommodate the higher compression, but with improved materials and injection technology, automotive diesels are quite competitive with otto cycle engines in terms of specific power these days. Marine diesel engines can have pretty spectacular performance characteristics http://people.bath.ac.uk/ccsshb/12cyl/
     
  7. Sep 20, 2007 #6

    brewnog

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    Ah, thanks. Slip of the tongue.

    The higher compression ratios (and higher cylinder pressures) necessitate the core components to be stronger, and (generally) as a result heavier than a spark ignition engine of similar output. In addition, the traditional expectation of a Diesel engine is for much longer life, though this comes largely from the historical commercial background of Diesel engines.
     
  8. Sep 20, 2007 #7

    Ivan Seeking

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    Diesels have greater fuel flexibility. The first diesel ran on peanut oil.

    Diesel fuel has a higher energy density than gasoline
     
  9. Sep 23, 2007 #8
    True!

    Not true! The first engine Dr. Diesel designed ran on coal dust, but he couldn't make the injection system work reliably, so he tried another alternative fuel: vegetable oil.

    I have 3 diesel vehicles: '01 Jetta TDI, '82 Mercedes, and '86 Isuzu. They all 3 run on high percentages of vegetable oil. The VW has been a vegetarian for almost 20k miles now.
     
  10. Sep 23, 2007 #9

    mgb_phys

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    The diesel engine was invented and first built by an English guy called Stuart.
    Fortunately his name didn't catch on or we would be talking about a Turbo-Stuart-Intercooler or Stuart-Dykes.
     
  11. Sep 23, 2007 #10
    I thought Stuart was an Aussie.
     
  12. Sep 24, 2007 #11

    Ivan Seeking

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    http://www.ybiofuels.org/bio_fuels/history_diesel.html

    So it is a matter of how you define "first".

    Is it even possible to get 75% efficiency out of a diesel? He must have been running a fantastically high compression ratio if true. I think the best that I've seen in large generator applications is just over 60%. This was based on fuel consumption rate, fuel energy density, and a generator driving a known electrical load. If we take the generator to be 90% efficient including the engine coupling, then the engine would be operating at about 67% efficiency.

    Ironically, peanut oil isn't biodiesel in the literal sense. Biodiesel is a fuel that can be made from just about any plant oil.
     
    Last edited: Sep 24, 2007
  13. Sep 24, 2007 #12

    Ivan Seeking

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    Fantastic! Do you mix vegey oil with diesel fuel directly?

    What is your normal range of ambient temperatures? Do you have trouble starting the engines in cold weather?
     
  14. Sep 24, 2007 #13

    mgb_phys

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    Last edited: Sep 24, 2007
  15. Sep 24, 2007 #14

    mgb_phys

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    Nay lad e wer a yorkshire man. Although he lived in Oz and built engines there.
    There isn't a monument or anything because the English don't like inventors ( Turing invents the computer and gets a mini-roundabout named after him) but some company in Halifax named a model of parking bollard after him!
     
  16. Sep 24, 2007 #15

    brewnog

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  17. Sep 24, 2007 #16

    mgb_phys

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    This is the most efficent and most powerful internal combustion engine http://people.bath.ac.uk/ccsshb/12cyl/
    There is a GE engine for power generation that is slightly more than 60% efficent but it is a gas turbine + diesel combined heat and power unit - since you use the hot exhaust for heating you get more efficency than if you just counted the shaft horse power.
     
  18. Sep 24, 2007 #17

    Ivan Seeking

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    I will dig up some links later. IIRC I was looking at large, two-stage diesel turbine generating stations [1 MW+ range]. When I got to the very large systems, there was a big jump from the typical 40% efficiency that I see for smaller engines.
     
    Last edited: Sep 24, 2007
  19. Sep 25, 2007 #18

    Ivan Seeking

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    I did some looking and didn't spot anything yet. I was thinking it was Caterpiller, but they have the 500 hp multi-fuel compatible, diesel turbine [an interesting find]. Their 3125 KVA gen units were only checking out at 42% total efficiency.

    I try again more later. I should have a link somewhere.
     
  20. Sep 25, 2007 #19
    I have a processing "plant" in my garage where the VO is washed, filtered, dewatered, filtered, blended, filtered, and pumped (while being filtered again) through a gas-station-type nozzle into the vehicles.

    I live in central North Carolina where the summer temps are around 70/90 and the winter temps are around 25/40. I blend with a concentration of D2/VO of around 20/80 in the summer and 60/40 in the winter. I do use block heaters almost every day of the year which costs probably around 20 cents per day. And I'm adding electrical fuel pumps and 12V injection line heaters right now which should allow me to increase my percentage of VO regardless of the season.


    Although it's clear he lived in England during his mid-years, I'm pretty sure he lived in Oz when he was young, but I'm not sure where he was born. We know he lived in Oz again when he was older. (Returning "home"???)
     
  21. Sep 25, 2007 #20

    mgb_phys

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    Last edited: Sep 25, 2007
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