The sfc thing has been bugging me, not having been convinced that such data would tell the whole story as regards actual fuel economy in real driving conditions. Good for a naked engine running at near full output behind a generator or water pump, but I wasn't sure about highway driving with a transfer case, U joints, differentials, and tires. so I placed a call to Cummins and asked how to predict or recommend a running cruising rpm in the interest of fuel economy. They said that this is impossible to predict, depends on many variables, and is best obtained experimentally for a particular rig, driver and driving conditions.

Part of the problem with studying our engines is that the turbocharger and ECM completely mask the engine's own inherent torque curve. that is, they chop it off (via turbocharger wastegates and ECM fuel regulation) to make it look real flat and impressive, which also results in that good looking linear HP graph. So the familiar dome-shaped torque curve has been pushed "upward" and flattened by use of magic. thus, the natural, mechanical torque sweet spot (torque peak rpm) is hidden. That doesn't mean it isn't there, however -- if the engine is at its mechanical torque peak rpm, lower boost pressures would be required to acheive full output.

When I asked about sfc curves, they told me that since 1997, Cummins has not published bsfc (brake horsepower specific fuel consumption) curves due to their irrelevance to highway fuel economy. Basically, they confirmed to me that the sfc curves are useful for continuous duty output applications at or near peak torque and are not useful for applications where the engine spends most of its time lightly loaded. cummins doesn't even have any data for the lightly loaded condition.

 

[quote author=Cat Cracker link=board=20;threadid=18317;start=15#msg172140 date=1060813116]
My truck seems to operate best between 65-70 mph. With the 4:10's that places me right at 2k rpm. I think thats the best compromise between "fuel efficiency" and "towing performance" Obviously, the slower you go, aka fewer rpm's the engine is turning, the better fuel economy you will get. (to a point) but you also risk hurting the engine by lugging it too hard. [/quote]
Thats what I tend to notice with my recent highway experiments. I can set the cruise on 55 and run in 6th gear around 1600. Or shift into 5th and run at the same speed at 2200. I get about 2 mpg worse at 2200.

But another thing I notice is that when I push the rpms down to closer to the lower limit of peak torque, the mpg is good but the "sensitivity" is real high. in other words, I can get very good instantaneous mpg cruising but the benefit of that lower gear goes out the window real quick if I have to pull up a small grade. In that case, I get a mpg advantage by shifting down -- even if both gears place the engine within the advertized peak torque range. This experiement has netted 1-3 mpg advantage to pull at 2200 rather than 1600. 1600 is just too close to the lower limit.

for the mathematically inclined, all this suggest to me that:

1. the engine gets poor fuel economy when used at the lower end of its peak torque curve (1600). It appears be more efficient when burning smaller charges of fuel more often

2. Raising engine rpms too high causes a couple of factors to converge, making the engine loose efficiency somewhere at or near 2000. first, you have the HP versus speed curve, which rises very fast (a cubic). Then you have the engine's own inirtial forces which rise also but not as fast (a quadratic). So the combination of these makes fuel economy go really bad really fast with increasing speed and rpm. Experimentally, this appears to be in the 2000 rpm region

What this says to me is that the lower rpms (below 1700) are a disadvantage because the engine gets poor economy when working at its peak torque capability at low rpms. Higher rpms are also a disadvantage because the engine gets poor economy when running over about 2200 and vehicle speed gets high.

So thats the closest I can come to to explaining why guys feel they get better fuel economy when towing at about 2000.

 

Low RPM under load like a hill or towing can and will lug the engine as well as send your EGTS up. I always drop a gear if I am cruising at lower RPMS and starting up a grade...The egt's will drop with the higher rpm pulling in more air...air plus fuel = power... ;D

 

There is an overlapping point range when the motor is making its most power in terms of both HP and torque. It is just this given RPM range that I would describe as its "sweet spot." With the law of diminishing returns in place, one might conclude that after a certain RPM and/or drag coefficient is obtained, the MPG goes down as does the "sweet spot" window. If the motor was able to constantly make more power to overcome drag as the RPM's increased, then it would always be in its "sweet spot"...wishfull thinking on my part!

 

For me the sweet spot is where the rig feels good. Just cruising down a level highway with no head wind it is 1750 rpm. When pulling a heavy trailer up a grade it is 2200 rpm, just seems to be having a comfortable workout without excessive EGTs.

 

[quote author=turbov6joe link=board=20;threadid=18317;start=30#msg177632 date=1062065561]
There is an overlapping point range when the motor is making its most power in terms of both HP and torque.
[/quote]
I think you're on the right track but technically this isn't true. The torque curve is flat from 1400 to 2700. this makes the HP function perfectly linear within this range, gradually rising from 1400 in direct proportion to rpm. So from that standpoint there is no sweet spot and HP is the highest at 2700. that aside, the "peak" HP capability occurs at 2900. Even though HP is directly proportional to rpm, the maximum torque capability decreases from its peak (flat from 1400-2700) value to something less at 2900 and the product of rpm times torque reaches a peak at 2900 before the torque spec goes way way down above 2900.

So the published HP curve of the engine is not the way to predict a sweet spot, simply because we don't drive at 2700.

However, I agree with what you are saying in that just because the turbo wastegate and ECM magic hide the natural mechanical peformance of the engine doesn't mean it isn't there. I think some research into the mechanical behavior of the engine would reveal a torque curve similar in shape to the familiar dome-shaped curves of yesteryear.
agreed. the inirtial forces of the engine (piston acceleration mainly) rise as the square of rpm, so the engine becomes inefficient at high rpms. wind drag increases as the square of vehicle speed, and drivetrain friction increases linearly with vehicle speed so the vehicle becomes expensive to run at very high speeds, esp towing somthing big.
for the most part, and between 1400 and 2700 the engine does constantly make more power as rpms increased, and this behavior (accomplished by the ECM and the wastegate) I think causes people to ignore the likelihood of a mechanical sweet spot and to do "wishful thinking". Indeed, the engine appears to behave like an ideal power source! So the ECM and the wastegate synthesize or fake a perfect engine between 1400 and 2700.

 

I'm going to bed. Tomorrow I will be driving my BEAUTIFUL BLACK TRUCK at about 1900-2000 rpm's, cause that's where the SWEET SPOT is.

 

Doug,
I agree with you on most points; however, on a dyno graph the torque curve of my 2003 HO 6 speed comes up REAL quick, carries fairly flat untill it drops off quickly at RPM x. The HP curve also comes up quickly, carries quite a bit further than the torque curve, then also drops off rapidly (nature of the diesel motor.) My definition of the "sweet spot" would be where if one were to plot the HP and turque curves, the point at which the two meet at RPM x would be the "sweet spot." The motor is making its most HP and torque both at this point....best of both worlds if you will. Now, this RPM point might not be the most fuel efficient, or be giving off its best sound, or make the truck ride/feel as smooth, but for all out power while towing or climbing a grade, this is the "sweet spot" IMHO. Anyhow, back into my world...cheers!

 

well, we have been talking about various definitions of the sweet spot haven't we ;D. I was trying to define the one mentioned in the article that started this thread! (fuel efficiency). What you're describing is peak HP, which is certainly sweet indeed...

A few thoughts on your post:

torque and HP curves don't ever "meet" on a graph because they are plotted on different graphs (scales). in other words, it is a meaningless concept to suggest that torque and HP meet somewhere because they don't. ever. Torque is plotted on a scale of 0-~600 and the stock trucks peak at 555 (flywheel). HP is plotted on a scale of 0 to ~350 and the stock trucks peak out at 305 (also flywheel).

HP is simply a calculated quantity that depends on the product of torque and rpm (literally torque times rpm). So the HP curve doesn't tell us anything more about the engine than a torque curve does, although it does present the same data in a different and sometimes more meaninful way. Peak power is a very important number, which the HP curve exposes very nicely, but it can always be obtained from the torque curve.

Also, keep in mind that the torque curves in our engines have been artificially flattened by turbocharger wastegates and ECMs, so the natural, efficiency sweet spot of the CTD mechanics is hidden. Dyno measurements of peak torque and HP will reflect this, and they too hide any natural mechanical efficiency behavior of the engine itself, quite apart from the external controls imposed.

peak HP and peak torque will never occur at the same rpm. The place where an engine develops the most power is at the HP peak (2900 for our engines) and that is always beyond the rpm where peak torque is, for both gas and diesel engines. thats just the nature of the mathematical calculation of HP, which says that HP is directly proportional to torque and RPM, without regard to what engine technology we're talking about. if rpm goes up by 10%, HP goes up by 10%. if torque goes up by 10%, HP goes up by 10%. If one goes up and the other goes down (by the same percentage), then HP stays constant. The magic of peak HP always stretching out further than peak torque is simply because, for the region just beyond peak torque, rpm always goes up faster than torque goes down -- and this is true for all real automotive engines, gas or diesel. for example, the CTD has about a 1/2 percent drop in torque between 2700 and 2900, but the change in rpm (200) is about 7.5 percent, so guess what: the math says that you get about 7% more HP at the higher rpm, and thats why peak HP occurs at 2900 rpm, while peak torque occurs at 2700. beyond 2900 of course, the CTD torque curve drops off FASTER than rpm rises, so you don't get any more HP out of the engine above 2900.

The HP calculation was developed (in part) to show the rpm at which the engine is capable of doing the most mechanical work (pumps more water, generates more electricity, tows more trailer up the same grade, etc)-- for all out power, as you say. For our engines, thats 2900 at WOT, and the CTD will do that all day. That's what you are describing as a sweet spot, and I have to agree it's very nice! You'll get better fuel economy though if you pull at 2000 ;D

 

Someone above mentioned that sfc values are not readily available. They are, for some engines. Cummins, for whatever reason, chooses not to publish sfc's for it's engines, but lots of other manufacturers do: http://www.sae.org/ohmag/features/ma...mahlechart.pdf

These are not sfc "curves"; just published values, usually at some rpm, and chosen, I'm sure, by the manufacturer because that was the lowest value available. I have seen sfc curves, taken from actual engines, in a textbook for an engines class that I took in college, which plot sfc vs. rpm over the operating range of the engine. The ones published in that book were, for all practical purposes, flat. They may have varied 5% from high to low over the rpm range of the engine.

 

Well after all thats the order of magnitude we're talking about -- 1 mpg in 20. That is, from the engine only. the other factors of speed and gearing are what make up the sweet spot.

 

No, that's not what you're looking for.

Let's say, for talking purposes, that the max variation is 5% over the range of rpm which encompasses, say, 40 mph (worst sfc) and 60 mph (best sfc). This would mean the engine uses 5% less fuel per horsepower per hour when travelling at 60 mph as compared to 40. By driving 60, the engine would be operating 5% more efficiently for each horsepower it produces. But, just due to aerodynamic drag alone, the engine must produce 3.375 times (60 divide by 40, then cubed) as much horsepower to go 50% faster.

For sfc to have an effect that could cause a mileage 'sweet spot', like an airplane has, it would have to be something like an inverse cubic function. It is nowhere close to that.

This has been an interesting thread.

 

fuel consumpton performance is specified by Cummins for the Dodge ISB -- just not the sfc curves. that is, you can obtain more information for our engines than is contained for other engines in the pdf you refered to -- fuel consumption in lbs per hour at three data points are available and all are at the engine's highest output (WOT). The datapoints are 1600 (lowest rpm at peak torque), 2900 (HP peak) and governed speed. What Cummins doesn't publish (that was my statement) is actual sfc curves, as you say, and the fuel consumption data they do publish doesn't tell us anything useful for the sweet spot discussion.

as for the so-called sweet spot discussion, I thought we were talking about an attempt to describe engine efficiency over an rpm range while keeping its power output rate (HP) constant. If all we're considering is changing vehicle speed to find an efficiency sweet spot, then absolutely, the cubic function of HP versus speed will dominate and the whole notion of a sweet spot is meaningless.

However it the engine is more efficient at a certain rpm, for a given rate of work output (HP) then the driver has more knowledge to apply in choosing gears. Its really more practical of a concept for 6-shooters towing loads (and/or those with outboard overdrives), where the driver has great control over engine rpm. simply choose the lowest acceptable speed that you can acheive near the targetted rpm. 4-speed slush boxes would rarely if ever see an practical opportunity to exploit this -- the spread between gears is just too large.