All Diesels idle with low fuel consumption

[Norm]

.....as one person posted about running a diesel on 3 cylinders....

     First of all I used to work on Diesels (mostly Gray Marine 671's and 4 cyl. Buda ) the injectors are designed to give maximum fuel efficiency at all speeds even ignition timing in a diesel is completely related to when the fuel is delivered.  

     When a diesel is running an injector is giving it just enough fuel to keep it running.

     A number of people can't get over the idea

that stems from running a car back in the old days when you had a gas engine and the carburetor was related to intake manifold vaccum the distance of the ports from the carburetor,(yes...that's still a small concern even with fuel injection on gas engines nowadays unless you have port-injection) You had to have a richer mixture when idleling plus the fact that in a car when you're at a stop light ....you are not moving you're wasting fuel...no matter how little you're using.

     I'd be almost willing to bet that if you had a small 4-cyl. diesel running at idle it wouldn't use any more fuel than a one cylinder diesel running at a speed to match it's power output.

   I'm mostly submitting that if you can get a small 4-cylinder diesel that it would be a waste of time trying to make a lesser engine of it.

            Norm.    

 

[bob g]

you are absolutely correct.

a diesel engine will burn around .35 lbs of fuel per horsepower per hour, give or take a bit. so:

it doesnt makes little difference if you have 4 cylinder making 10 hp or 1 cylinder making 10hp.

and once again i would not recommend disabling any cylinders on a 4 cylinder diesel, it is not the same thing as a gas engine as the compression ratio on the diesel is roughly  2 to 3 times that of a comparable gas engine. You are running the very real risk of breaking a crankshaft!

disabling injectors on a direct injection system is not just a simple matter or unplugging wires either.

i have been a hd diesel mechanic for 30 years now, and have certs for detroit diesel, cummins, cat, and mack, i also work on a variety of smaller import diesels, such as isuzu, mitsu, volvo, etc.

if you want trouble, just start removing pistons, and disabling cylinders.

my opinion for what it is worth

bob g

[wdyasq]

The only other thing that might matter is the energy used just to turn the running parts.  A smaller engine has less parasitic drag than a larger engine and will therefore use less fuel at lower speeds.  

All engines have a speed they are most efficient running.  A little efficeincy can be gained by running an engine in the best conditions of load/temperature/speed.

The "thumbnail" figure on diesel fuel consumption I learned was "20hp/gal/hr".  This means a diesel burning a gallon of fuel an hour wil be developing about 20 Hp.

Bob G's .35lb/hp/hr works out to 19.54 hp/gal/hr - close enough for my guessing.

Ron

[brock]

Yup, with using a laptop you can find a VW Jetta gasser 1.8 at idle uses about .9L per hour.  While a VW Jetta diesel 1.9L (TDI) run about .3L per hour.  Both just at idle.  In this case, they are both modern engines and the diesel is consuming about 1/3 that of the gasser.  This amongst other things is how we can get 50+ mpg with the TDI's.

[bob g]

i would agree on running the engine at its sweet spot so to speak, based on loading, etc.

the parasitic losses are an interesting thing tho' for instance

take two engines, one 4 cylinder of 100 cubic inches, the other a single cylinder of 25 cubic inches, same bore and stroke, same basic design.

the single cylinder theoretically will have to run 4 times faster to develope the same horsepower as the 4 cylinder. at the higher rpm the parasitic losses will be around 4 times higher, so it ends up as a wash.

what is interesting to note, an example used in one of my engineering books, took two constrasting engines, on a small single cylinder diesel of around 10 cc, as opposed to a giant diesel multi cylinder engine used in an ocean going ship.

the difference in consumption per horsepower/hour was within .02 gallon per hour per horsepower.

i will look up the reference and post it, it was very enlightening to me, and quite facinating to say the least. i would have expected a much wider variation.

bob g

[pat]

This site gives engine performance for Kubota diesel engines:

http://www.kubotaengine.com/kubotaperformance.htm

From these interesting charts, you see that larger engines use about 0.39 lb/HP-H and smaller ones use 0.41. Not much difference to worry about.

You are right Norm, larger engine tends to use less fuel per HP per Hour for the same output. Given a stationary engine application, I would say that an engine a bit larger is better. It make sens to use a 50 HP to produce 10 HP at idle, of course it would note make sens to try to produce 10 HP with a 400 HP power plant. I think it would be impossible...

BTW, What's the slowest RPM you could run a diesel engine at?

Thanks

PAT

[bob g]

most diesels dont make much heat below about 1200 rpm,

for a 4 cyl engine i would run it over 1200rpm at a point where vibration is at a minimum

bob g

[Gary D]

This is very interesting.... seems our so called experts in gas hybreds are purposly barking up the wrong tree so to speak. wouldn't be the first time! Of course some dainty ladies might have objections at filling up at truck stops... Diesel electric seems better to this dummie.... Gary D.

[JB]

Hey Bob maybe you can give me a pointer. The original heavy fuel line on my allis hd 4 dozer was leakin. The one that goes from the tank to the filter. I replaced it with a larger hose ,regular automotive fuel line that I had . Now when I start it it revsup  maybe 20 seconds or so and then comes back down to idle. Its not good and drives me nuts. I cant seem to see any air leaks and it seems like maybe there is air . I bled the filter. It has a roosamaster and it does this cold or hot. Got any ideas. Thanks JB

[bob golding]

very interesting, going on this  i might just as well use the 8.5 ltr 6 pot Bedford 500 engine in my bus to run my alternator instead of the 2.5 ltr ford. would be easier and i get the hot water as well. my landlady would sure like it if i had one less heap of scrap  lying around  ;-). just to  help run the numbers i want to use a 2300 watt alternator to run my  new 100 amp battery charger charging a 800 amp battery bank.

bob golding

[Norm]

   I don't know how slow you can run a diesel there too ...other considerations a single-cylinder like a lister with a heavy flywheel pretty slow....used to have a little 1hp. Briggs that we could actually count the revolutions by eye...I imagine a Lister could do the same...

   BTW Would a 2-cycle diesel be just as good on fuel as a 4-cycle?

            Norm.

 

[pat]

Norm,

That's a good question! Two-Stroke diesel engines are normaly huge so I don't think that anyone would use them for Homebrewed Electricity !

I found some good info about these engines at:

http://auto.howstuffworks.com/diesel-two-stroke.htm

PAT

[wdyasq]

Both 2 and 4 stroke diesels are built in all sizes. I think 2 strokes might eb built a lot smaller than any 4 stroke.  I personally have held half a dozen 2 stroke diesels in the palm of one hand.

Ron

[nothing to lose]

As a thought, would those little COX type airplane engines be small diesels?

You know like little radio controled airplanes run. Always wondered what that fuel is? Not gas, not diesel, but they do run glow plugs. Must be something like a better grade of diesel, or perhaps it was corn oil?

[bedlamite]

Most model engines run on a mix of methanol, 15-20% castor oil, and 5-30% nitromethane. They also have compression ratios from 7:1 to 10:1, too low for diesel, but there are conversion kits available for some engines so you can run diesel. Model diesel fuel is a mix of kerosene, 2% MEKP, 25-33% ether, and 25-33% castor oil. Both fuel efficiency and power are better with diesel than nitro, but fuel efficiency is still still not really that good.

[Norm]

  As I recall even mocel airplane have heavy flywheel kits for use on model cars and boats.

  I remember back in the  '40's when they had model engines called 'Deezil's' you could change the compression ratio by screwing down a small piston at the top of the cylinder.

   Crossed my mind....I don't think these little 'screamin' demons' really have an idle speed ...do they?          Norm.

[nothing to lose]

That little Cox model plane engine I had just ran fast as you set it. It had a screw for something, feul flow, idle  or what ever, but it never actually ran slow, it screamed till it ran out of feul! You could just control how fast it screamed!

I only used it a few times. Don't remember too much about em.

[bedlamite]

The screw on the Cox engines is for air/fuel mixture adjustment. Cox and a few others just run wide open to save the weight of a throttle servo, but most model engines have a carburetor with both high and low speed mixture adjustment, and an idle speed set screw. You get around a 10:1 ratio for max/idle speed; With a 15K rpm max, you should get a 1500 rpm idle.

[windstuffnow]

  If you look closely at the so called hybrid technology you'll find what they're offering the general public is an agency controlled fuel efficiency.  The honda's and toyota's are getting around 50mpg.  They are also charging extra because they say its "new technology".  Look around the world at what others are building and driving.  The VW bug diesel gets 45 to 48 mpg, others around the world are getting 50 mpg or more some on gas.  None of them have the hybrid technology.  Watch the ad's on TV, pay attention to what they are pushing on us.  Do we have a choice?  Bigger vehicles, bigger engines and worse fuel economy.  GM cut their S10 line completely to start pushing the Colorado - bigger truck - bigger engine.  No more 4 cyl trucks.  Ever notice there are no 4cyl diesels available in the american car or truck line up?  They get way to good of fuel economy... can't have that.  The 1981 chevy Luv truck with a diesel would get over 45mpg and a similar version was installed in an S10 achieving similar results.  Only one year for the S10 and never offered again in anything.  Do we control the market or does the market control us?   Personally I won't buy anything they build until they build what I want... or I'll build it myself...

Food for thought

Windstuff Ed

[walsdos]

Anecdotal on diesel fuel consumption:-  Having owned a few  and found varying rates of SPECIFIC fuel consumption here are some results.

    David Brown 4 cylinder direct injection 40 HP "approx 150cu" 29HP/ Imp. Gal on test bench.

   Perkins 6 cyl 354 cu ran, @ idle, 14 hours on 1 Imp Gal

   M A N  27,000 HP 9 Cyl 125 RPM  27 HP/ Imp Gal on heavy crude with ash removed.

 I also turbo-charged a Zetor 4 cyl 200 cu 60 HP engine to 12psi with no mods to fuelling and had a dramatic power increase. 1 to 2 gears higher , but no dyno test.

  That was a learning experience since the rings rolled out of their grooves and Zetor hadn't heard of turboing toys until then so no appropriate pistons

[Xavy]

I was wondering if you could make a comparison of a fuel usage relation between driving on level land and gearing down with a heavy load going down a steep grade the same distance.

The reason I ask is it seems to me the energy used for backpressure and braking could be transferred into a generator as brake HP.  I heard about early trolleys that used a generator with the "field" controlled by a rheostat.  When the trolley required stopping resistance was added to the generator field until the brake HP equaled the load.  The generated power was transferred back to the grid for other trolleys to use.  Thought such a system could be devised for trailer trucks going down grades since mountains are all over the world.

Xavy  

[schultz]

They seem to fit the rules of thumb that have been pointed out.  Here is a web site for a 200 hp 2cycle diesel airplane engine.

http://www.deltahawkengines.com/specif00.htm

THis engine uses .39lb/hp/hr