are gas engines about as efficient as diesels?

[bigtoy]

Or am I being fed a line of bull? A friend of mine stopped over last night to show me his new to him 02 chevy reg cab with the 5. 3 liter gas. We went for a drive and stopped to talk to a guy that has a new ford 150 4 door gasser. They commented that they each were getting over 18 mpg while driving 75-80 mph. Are these new gas engines getting that good of mileage or are they full of it? These trucks are both 4x4 to boot. The best mileage I have ever got with a 4x4 gas was with a 95 chevy reg cab with a 305 V8 driving about 60 to 65 mph. I got just over 16 mpg. Thats pretty good mileage but you have so little power with a motor like that. The best mileage I have got with my CTD is 21 on a 350 mile trip. Short trips usually 15-18 mpg. If these gas owners are stretching things by 2-3 mpg, then I will have to raise my mpg to about 23-24 mpg!!!

 

[RustyJC]

From an article at prime-mover.org

Typical BSFC ranges are as follows. As usual, there will be exceptions, but this should serve as a general guide:



0. 26-0. 34 Large industrial four-stroke diesel engines (very small hp/weight ratios)



0. 28-0. 36 Other four-stroke diesel engines



0. 32-0. 38 Two-stroke diesel engines



0. 37-0. 44 Fuel injected four-stroke gasoline aircraft engines



0. 40-0. 48 Fuel injected four-stroke gasoline automobile engines



0. 43-0. 48 Carburetted four-stroke gasoline aircraft engines



0. 48-0. 60 Carburetted four-stroke automobile engines



0. 55+ Two stroke gasoline engines



0. 55-0. 70 Four-stroke aircraft engine takeoff fuel flows

So, based on the above (as one would expect), fuel injected gasoline automotive engines are not as efficient as diesel engines.



Rusty

 

[Mhannink]

gassers are less thermal effeicent then diesels thats one reason why there milage is not as good they waste alot more heat then a diesel does lol it all stems from the spark plug

 

[Dieselnut59]

I have heard that all things being equal, you can take two similarly powered engines, one gas, one diesel and the diesel will get 30% better fuel economy on average. The Mercedes E300 CDI is proof of that. Take the same car, equipped the same way and the diesel in it outscores the gasser in EVERY way. It gets WAY better fuel economy (37hwy mpg versus 28 for the gasser I believe), has better quarter mile time and better 0-60 mph time. This is with a 3. 2L gasser and a 3. 2L diesel with auto trans. in the E320 car. I don't remember where I saw the article, but I have been studying up on this car for a while because my mother is interested in one.



The figures those guys gave you with their "half ton" gassers are "not bad" by today's standards, but if you were to take that exact same truck and equip it with a similarly powered diesel, I would say you would see MPG figures around 25-28 or better.

 

[wxman]

Here are more citations from references I've run across recently:





"… Diesel engines are already more efficient than gasoline engines (45 percent versus 30 percent), and further advances are possible (to 55-63 percent)…. "



Source: http://www.eere.energy.gov/vehiclesandfuels/pdfs/basics/jtb_diesel_engine.pdf





"…An evaluation of 24 matched pairs there [in Europe] found that…turbocharged, direct injection (TDI) diesel vehicles averaged 50 percent higher fuel economy…. "



Source: Deliberating Diesels: Environmental, Technical, and Social Factors Affecting Diesel Passenger Vehicle Prospects in the United States, ACEEE, Report Number T032, James Kliesch and Theresa Langer, September 2003





“…With all other things being equal, today’s diesel engines are projected to achieve up to 40% higher fuel economy than today’s gasoline engines, which is equivalent to about a 29% savings in fuel consumption. …”



Source: http://www.epa.gov/otaq/technology/420r05012.pdf





“…On average, diesels consume 34 to 40% less fuel per mile than comparable gasoline cars. (50 to 70% more mpg)…”



Source: http://www.eere.energy.gov/vehiclesandfuels/pdfs/deer_2005/panel2/2005_deer_johnson.pdf





"… Cummins is maintaining the tremendous fuel economy advantages of its ultra-clean light-duty diesels--60% better fuel economy versus the comparable gasoline engine…. "



Source: http://www.findarticles.com/cf_0/m0CYH/16_7/107756429/p1/article.jhtml

 

[blacksheep4x4]

My buddy has a new F-150 and he can't get the truck to do more than 11mpg.

 

[JUeckert]

generally speaking under like conditions diesels deliver better fuel mileage. gasser's have closed the gap over the years however. i do know for a fact that the 5. 3 gasser is capable of some impressive mileage. my dad has a second truck with one in it,23 mpg is easy on the highway,and runs great as well[2wd]. uncle just bought a ford 1/2 ton with the 5. 4,he claims 21mpg on his last visit.

 

[Tony T.]

Theres a guy at work that bought a new 05 Nissan Titan and he was not very impressed with the mileage on that truck, of course I have no way of knowing his driving habits but he was telling he was only getting 13 mpg hwy.



Tony

 

[EricBu12]

Ok heres another thing. The Gasser you refer to is a regular cab and you probably have a quad cab... and the Diesel is heavier (my block weighs 500#) and so if on long trips you are about equal, the weight of the gasser regular cab will be way less than the dodge...



My truck weighs almost 8000 # solo... ..... How much does that regular cab gasser weigh? 5000? if that?



Diesels get 40% better milage on average on equal ground ... ... Diesel has a higher BTU in the higher compression ratio. ..... now, right now the Diesel Fuel is 20% higher price so you are getting a 20% savings in the long run.



My dad had an old Ford Club Cab with the 460 Ci Carburated. He got about 6 MPG... !! Now the gassers have gotten better milage over the years, why? Fuel injection, High energy ignition, computerized controlls, and higher compression.



So yeah, your buddy might have good mileage, but can he haul a heavy load up a hill at more than 50 mph? I'd say maybe but he will be revving high pushing the Gas hard. And take heart the Diesel will last about 3 times longer than a gas engine.



I took a long trip solo one time and I was able to get 20-23 MPG running 70-75. I have a little larger tire so the revs were like a 265 tire at 65mph.

I have a 15K 5th wheel and i went down to Monument Valley... High heat and steep hills. I live in Salt Lake City so I am in the mountains alot. I average 10-12 MPG towing this beast... ... see if the gassers can top that!!

 

[BSkinner]

My 2001, 5. 3 GMC averaged 14 mpg for all fills (and was a real dog, especially trying to pull itself up the mountain). My 2003 3/4 ton Dodge diesel has averaged (86 fills-since new) 17. 8 mpg. Very little light towing with either

vehicle.



Bob

 

[Cattletrkr]

My brother's '99 chev 1500 w/5. 3 broke the 20mpg mark once. It was coming across WI with a heck of a tailwind, but it still counts. I was with on the trip and saw the actual #'s myself, no BS. He's gotten 17mpg pulling his little boat, but rarely gets any better empty.



A friend has a '97 chev 1500 w/350. He routinely gets 17mpg empty. Don't know what he gets pulling his boat.



I'll take a chevy over a ford for mileage any day. I've never met a F150 owner who's happy with his mileage.

 

[rbattelle]

Somewhere, (probably in Charles Fayette Taylor's book, The Internal Combustion Engine in Theory and Practice), I read that at the same compression ratio the spark ignition cycle (the Otto cycle) is more efficient than the Diesel cycle. Obviously, this is a meaningless "victory" for spark ignition, since KMEP is far lower than it is in compression ignition engines.



Rusty - can you corroborate that at all? Of course, I can't find the discussion in Taylor's book now.



-Ryan

 

[cojhl2]

It would seem that because the diesel by design follows more closely to the Carnot cycle it cant help but to be mor thermal efficient.



For the same compression, ie SI values I would believe that diesel maybe is not as effective and efficient cause the fuel cnt fire spontaneously but has to follow a propagation path. In the hi comp of a CI engine the fuel fires without regard to oxygen levels (so long as there is enough O) and no matter where it is in the combustion chamber.



Be interesting to fine that in the book Ryan. I have an old one somplace I should find it and look also.

 

[bigtoy]

Interesting! Thanks for the replies!

 

[TWilkening]

The truck in my signature will get over 18 MPG on the highway. It has 4. 10 gears. On average, I get around 16 with it.

 

[nickleinonen]

my pop has a 99 1500 silverado with the 5. 3 liter engine. he tows his work trailer almost all the time, and his combined city/hwy economy is about the same as my average city economy. . he drives like an old granny though, and i drive 75% of the time with the foot going through [at least attempting it] the floorboard.

 

[MPMurphy]

Diesel/Otto Cycle Comparison

If using an engineering text comparison between the Otto and Diesel cycles, one must be careful. The Diesel cycle is similar to the Carnot ideal heat engine cycle in that the pressure in the cylinder never exceeds the compression pressure achieved before combustion starts. Fuel is injected to maintain that constant pressure until injection of fuel stops. Modern "Diesel" engines do not run on that cycle. Our modern "Diesel" engines run on what some term a dual cycle, as once injection starts, fuel is admitted at such a rate that causes cylinder pressure to exceed the level achieved by compression alone, just as in the Otto cycle. Near its peak during combustion, the cylinder pressure can flat top, and become constant as in the Diesel cycle thus the "Dual Cycle" moniker. True Diesel engines running on the cycle of the same name are eerily quiet due to the lack of violent change in cylinder pressure that cause mechanically transmitted knocks and bangs to shake the air around the engine and make the noises we are used to hearing. Even though his cycle was a dead end and runs today in a small number of engines in museums, his name lives on into history! The guys and companys that really made the oil engine what it is today are utterly forgotten, like Akroyd and the de la Vergne company... Also, our oil engines expand the hot gas further, extracting more energy from them than an Otto cycle engine. Since mixture ratio control is not required in a compression ignition engine, most oil engines do not have throttle plates that create large pumping losses on the intake stroke. With a vacuum in the intake manifold at part load, a spark ignited gas engine needs more power to pull the piston down against the difference between manifold pressure and atmospheric pressure on the crankcase side of the piston. Finally, an exhaust driven turbo-supercharger helps to recover some of the energy in the hot gas before it is lost out the tail pipe and uses that to cram more air into the engine and help push the piston the right way during intake. The last three effects help "Diesel" engines gain some efficiency advantages over spark ignited, non turbo-supercharged Otto cycle engines. Sorry for the long post, and the situation is more complicated than I have made it sound, but these are some of the basics.

 

[rbattelle]

Modern "Diesel" engines do not run on that cycle. Our modern "Diesel" engines run on what some term a dual cycle, as once injection starts, fuel is admitted at such a rate that causes cylinder pressure to exceed the level achieved by compression alone, just as in the Otto cycle.

Excellent point. Great post.



I found the text I was thinking about. It wasn't Taylor, it was a book by Moran and Shapiro called Fundamentals of Engineering Thermodynamics (3rd ed). In it, they show that the cold air-standard analysis of the Otto cycle and the Diesel (combustion at constant pressure) indicates higher efficiency in the Otto cycle for equivalent compression ratios. They also point out that the dual cycle is a much better description of the actual cycle than the Otto or Diesel cycle are.



-Ryan

 

[MPMurphy]

Thanks, Ryan...

 

[JessePayne]

I averaged 20mpg in my 01 ram towing my boat for 400 miles each way a couple of weeks ago