Engine HP v. mileage

[BG6]

In another discussion, someone suggested that a big engine would get poor mileage.

This is not true -- at least, not for any engine you are going to see on a coach.

It takes a certain amount of power to move a particular mass at a particular velocity.  It takes a certain amount of fuel to get that horsepower.

Let's say that Engine A is a DD Series 60, set for 500 HP.  Engine B is a Cat 3176 (used in a lot of diesel pusher high-end S&S RVs) that gets 365 HP.

Let's also say that the loads are identical, the only difference being which engine is in which coach, going on I-80 between Salt Lake City and Sacramento, then turn around and go back.

Going across the salt flats -- no road in the world is flatter -- both will get the same mileage IN STILL AIR.  The engines have the same amount of work to do.  It doesn't matter how big the cylinders are that do that work.

However, the moment the load changes -- headwinds, hills, variable speeds for traffic, etc -- the extra horsepower starts saving you money.

This lesson was learned the hard way 10 - 15 years ago.  A lot of truck companies put in smaller engines after the Exxon Valdez accident made fuel prices jump.  That 3176 engine went into thousands of semis, owned by fleets nationwide.

What they discovered was that the majority of fuel is used to accelerate, and that the less time spent accelerating, the sooner the driver could get into the fuel-efficient RPM range.

They also discovered that an engine with enough power let the driver stay in the higher gears, thus lower RPM, while going up hills or getting back up to speed after slowing for traffic.

Think of the Apollo-Saturn launch system that took us to the Moon.  A massive amount of fuel was expended during launch, followed by days of coasting.  This burned a fraction of the fuel which would have been needed for a constant acceleration, turnover, then constant deceleration. 

Your coach is the same way.  Those two or three pedals are the keys to someone else's cash register.  Every time you touch one, you spend money, either for fuel or for brakes and clutch.  It used to be said that every time you push the clutch pedal cost a penny in worn lining and pressure plate.  I hate to think what it costs now, but I "float" my shifts whenever possible.

Fuel is the most expensive item over the life of a semi.  An owner may spend $500 PER WEEK on the truck payment, but if he's driving 3000 miles per week, he will burn over 400 gallons of fuel!  If having a big engine was a bad thing, you would see Freightliners using Briggs & Stratton pull-start engines instead of 500 and 600 HP Detroits! 

[bevans6]

I think you're right, for identical engines, but engine efficiency can make a big difference to fuel usage.  Only in theory does an identical amount of fuel produce an identical amount of energy in different engines.  Breathing efficiency, compression, cam timing, injector profiles and timing, turbo, intake and exhaust manifolds, and drivetrains all have big impacts on fuel usage.

The big downside to a high power engine is if you use the power you use more fuel.  The driver makes a huge difference to fuel usage with his foot and what he does with gears, and how he manages terrain.  If you never used the power advantage, a big engine wouldn't need more cooling that a little engine.

Brian

[luvrbus]

The low rpm torque curve is where fuel saving come into play you can have a S60 without the right gearing to run in the 1400 rpm range and running at 1800 rpm will use as much fuel as 8v92 running the same rpm.
I know to many people that have Cummins, Cat and series 60 that I got better mileage with a 8v92 than they did because of gearing


good luck

[BG6]

I think you're right, for identical engines, but engine efficiency can make a big difference to fuel usage.  

Not as great a difference as you might think.

The big iron diesels these days are within a couple of percent of each other, no matter the make or model, due to computerization.  This is down below the noise level, where your tires, drive train, etc are a greater factor.

[BG6]

The low rpm torque curve is where fuel saving come into play you can have a S60 without the right gearing to run in the 1400 rpm range and running at 1800 rpm will use as much fuel as 8v92 running the same rpm.
I know to many people that have Cummins, Cat and series 60 that I got better mileage with a 8v92 than they did because of gearing

Yes, gearing is a big issue, but if I have a DD S60 at 500 HP, I can stay in my higher gear when that Cat has had to drop to a lower gear at higher RPM.

[luvrbus]

Install a C15 Cat it will dance with series 60 but a 3176 is not that big of a engine


good luck

[BG6]

Install a C15 Cat it will dance with series 60 but a 3176 is not that big of a engine

Tell me about it.  I sat behind one for two years as an owner-operator.  When anyone asked what I had for an engine, I proudly said "Briggs & Stratton, 3-1/2 horsepower!"

[RoyJ]

Install a C15 Cat it will dance with series 60 but a 3176 is not that big of a engine


good luck

Actually, 9 times out of 10 the big Cat would murder a S60 when hauling heavy. By trucking standards the S60 is pretty gutless down low, and Cats (or Cummins) shine down in the RPM range. Now the DD15/16 might be a different story, but I've never driven one.


Regarding hp vs mileage, it gets a little more complicated than simply saying engine "size" doesn't matter. Yes, bottom line is two identical coaches would require the exact same hp going down the road, but the difference in mileage comes from engine efficiency, or else OTR trucks would use blown HEMIs...

In a typical coach (where hp requirement is fairly low), a smaller engine, such as an ISM, would have higher thermal efficiency, due to the smaller surface area. The pumping / reciprocating losses would also be less, from smaller displacement/moving parts. At cruising, you're also much closer to the engine's peak efficiency load/rpm.

Now, if you're talking about underpowering a coach/truck, then a bigger engine would get better mpg, as you're pushing the smaller engine past its ideal efficiency envelope.

There's also quite a difference between a high hp small engine (450hp ISM) vs low hp big engine (425hp ISX). Here, the worst case is the "fleet setup", where they detune a big engine down to ridiculous levels. Reason being, to have a big block producing only 425hp, you'll be driving it much like a small block - revving way up. Then, you have the power of a small engine, but ALL the parasitic losses of a big engine (thermal, pumping, reciprocating, accessory drives).

Further complicating the issue, a big engine / high hp setup can sometimes get very good mpg, IF driven correctly. Guys with hopped up Cats making 900 hp often get better mpg than fleet trucks. My guess is that by having excessive hp, you can pull the same load at a much lower rpm. A 900hp Cat probably has a healthy 500hp at only 1300 rpm (w/ 2020 lb-ft torque). Pulling hills at this rpm would have much lower parasitic friction/pumping losses.

There're probably a bunch of other factors I didn't touch on. But in conclusion, is fair to say that just because a coach has a fixed hp requirement, doesn't mean every engine can get the exact same efficiency making that amount of hp.

[cody]

900 hp in my iggle would boggle me, to see the look on the corvette down the street as I do a burnout would be priceless lol.  Yeah, I know I'm dreaming but it still would be priceless lol.

[Van]

LoL! Now Cody you you know as well as I do... all it takes is dough   but seeing the look on the drivers face of that Vette when he realizes your still on his bumper... that's Priceless!  

[cody]

 

[BG6]

900 hp in my iggle would boggle me, to see the look on the corvette down the street as I do a burnout would be priceless lol.  Yeah, I know I'm dreaming but it still would be priceless lol.

You know, if you take out a couple of windows and add some duct, you could drop a jet engine with afterburner  . . .imagine the look on his face when you drive up to the stop sign, shut down the diesel, a set of doors in the back open and he hears the Pratt & Whitney spool up . . .

[Ed Hackenbruch]

Imagine the look on his face when you pass him and peel the paint off of that vet!

[buswarrior]

Lovely discussion, and fleetmen across the world play the variables in order to earn a few more pennies on the fuel budget...

But what busnut is installing a large engine and not dipping into it?

Blows it all out the window, for he with 500 HP will surely use more of it than is necessary for efficient movement down the road.

I'm buying a bottle to drink it, not put it on a shelf?

happy coaching!
buswarrior

[RoyJ]

900 hp in my iggle would boggle me, to see the look on the corvette down the street as I do a burnout would be priceless lol.  Yeah, I know I'm dreaming but it still would be priceless lol.

Well, couple years back, a co-worker of mine had a 1100hp Dodge Ram 5.9 Cummins. Even on street trim he had close to 900 hp. When he goes fishing in the summer, he has a camper on the back, and pulls a 14' boat behind.

He got to see a LOT of priceless looks on Vette and Stang drivers up the Coquihalla pass here in BC!

But what busnut is installing a large engine and not dipping into it?

Who said you can't dip into it? 

If I had a C15 with full marine goodies and Pittsburgh box, in my 24,000lbs bus, you can bet I'll let other drivers know about it! (and the tire shop...)

I like to buy the bottle, drink it, and let people know I've got the biggest bottle around