How little engine power is just barely enough?

[Chance]

Originally Posted by Silver350

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But I can tell you that my big truck will not go up a big hill any measurable amount faster in a lower gear at higher RPM. The difference will be equal to a diff ratio or tire size change.
It will however use more fuel.

Engine size versus design RPM is an interesting subject; although it can get a little complicated. You are correct in implying that engines are more fuel efficient at lower RPM. Typically the sweet spot on efficiency curve is at or below the maximum torque RPM. In your case the 5.4L V8 will use less fuel at 2500 RPM or lower than if you shifted down and climbed a grade in a lower gear at much higher RPM.

However, it gets a little complicated when a driver is after overall fuel economy when choosing an engine size. An example that comes to mind that I've been kicking around is this:

Ford rates the 4.6L V8 in the E-Series at 225 HP at 4800 RPM, and at 286 lb-ft at 3500 RPM. The 5.4L V8 is 255 HP at 4500 RPM and 350 lb-ft at 2500 RPM.

Assuming appropriate gearing is possible, the 4.6L produces more power (190 HP at 3500 RPM) at maximum torque RPM than the 5.4L (167 HP at 2500 RPM) at its maximum torque RPM. Therefore, a 4.6L V8 could do the same job you are doing by simply spinning faster in a lower gear during the hill climb in your example.

I don't doubt that your 5.4L would be more fuel efficient at 2500 RPM than a 4.6L at 3500 RPM, but what happens the rest of the time when cruising on level roads? What's the cost of running an engine that is too large and running it at lower partial load?

While engines run more efficient at lower RPM (maximum torque RPM or just below), the greater factor that affects fuel economy is how much load is on the engine. Gasoline engines need to be loaded at least 50% of available torque before they start to become efficient. By the time they get around 75% of available torque they approach maximum fuel economy.

If a van like yours had a 4.6L versus a 5.4L, it might use more fuel while towing a large trailer, but it's also likely it could save fuel when running without a trailer. So in a case like mine where I rarely pull a trailer, replacing my 6.8L V10 with a 4.6L V8 should save significant fuel most of the time. With the same gearing I could see an improvement of about 17% at slow cruising speeds (below 70 MPH).

Depending on how fast I wanted to drive, I could save even more by going to an even smaller engine. That's why new Transit and ProMaster vans will have V6 engines in the 3.6 to 3.7 liter range. A V10 like mine is very wasteful most of the time -- for my driving conditions anyway.

[Silver350]

I had a chance to do a little testing yesterday.

This is a 450/1550 Mercedes pulling 76,000# up a 6% grade:
Peak TQ @ 1050-1450 RPM
Peak HP @ 1900 RPM
Note the turbo boost to the right of the speedometer.


At an efficient RPM with lots of torque " in reserve" in 6L for a change in grade or slowing down for another vehicle etc:


Same grade at maximum HP in 5L:


A different but similar hill:
At the bottom of the torque range in 6L, usually only operate at this low RPM near the top of a hill when I know it will crest without a downshift.


And again in 5L pushing the engine to the max. This caused the coolant temp to rise so fast that I backed out of it right after taking the picture. I'd had never even attempted it going south on I-15 out of Nevada on a July afternoon.....

[Ford_6L_E350]

That's interesting, but I'm not sure how it applies to Sportsmobiles.

I've never found a hill I couldn't pull up at speed. I have found many that I can't go up at max torque. Long before I get near full throttle, my transmission will downshift. It seems Ford and the powertrain programmers believe I get better performance at higher RPM and closer to max horsepower than I do at max torque.

All newer Sportsmobiles have automatic transmission and we can't always choose the gear we desire. I don't even know when the last manual transmission was offered in a full size van.

Mike

[Chance]

Originally Posted by Ford_6L_E350

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All newer Sportsmobiles have automatic transmission and we can't always choose the gear we desire. I don't even know when the last manual transmission was offered in a full size van.

Mike

Could change soon. The new ProMaster diesels will have automated manual. What I've read suggests driver can select gear. Soon Sportsmobile owners may get to control gears more directly.

[jage]

Originally Posted by Chance

If I need only 50 HP to cruise at 70 MPH, I can lift off the throttle on my 305 HP V10 but that makes the engine run less efficient.

How so? By which I mean what exactly are you defining as efficiency; and if you consume more to reach an efficiency ratio, is that really better than consuming less at lower efficiency?

[Chance]

Originally Posted by jage

How so? By which I mean what exactly are you defining as efficiency; and if you consume more to reach an efficiency ratio, is that really better than consuming less at lower efficiency?

Efficiency is normally defined as a ratio of what you get out divided by what you put in. In rating engine performance we could use a percent of energy out (i.e. -- work done by engine) divided by fuel energy consumed (i.e. -- energy content of gasoline or diesel), but it's more typical to look at it on an instantaneous basis.

Mechanical engineers often define engine efficiency by what they call BSFC (brake specific fuel consumption) which in US units is stated as pounds per hour of fuel burned per horsepower. If you do an internet search you'll find what they call a BSFC "map", which shows engine efficiency at any given load and RPM.

In the case I estimated above, I know that if my van requires 50 HP at 2000 RPM, then the torque being produced is 131 lb-ft. That makes my V10 run only about 1/3 loaded which makes it burn a lot of fuel for just 50 HP.

If I had a 4.6L V8 running the same 50 HP at the same 2000 RPM, the same 131 lb-ft would make the engine run about 1/2 loaded. And BSFC maps for gasoline engines would indicate that the 4.6L V8 would burn less fuel than a 6.8L V10 at that particular 50 HP load and 2000 RPM.

Obviously the smaller engine requires more throttle input in order to run at 1/2 load versus the bigger engine running at 1/3 load (I'm using rough fractions without trying to be exact); but the end result is that it will burn less fuel and I'd see improvement in miles per gallon (under those conditions).

Hope that answers your question as to what I meant by lifting off throttle in my V10. In general gasoline engines need to be loaded down to become efficient, hence why higher gearing normally gets better MPG. Likewise why we use overdrive transmissions to make engine run slower and therefore increase required torque.

If I were interested in best MPG and was willing to drive at say 60 MPH on Interstates, I'd have to look at a much smaller engine than a 4.6L V8 in order to get best efficiency. Probably a 3 liter or so would give me best MPG at that speed on level road.

[Silver350]

Originally Posted by Ford_6L_E350

That's interesting, but I'm not sure how it applies to Sportsmobiles.

I've never found a hill I couldn't pull up at speed. I have found many that I can't go up at max torque. Long before I get near full throttle, my transmission will downshift. It seems Ford and the powertrain programmers believe I get better performance at higher RPM and closer to max horsepower than I do at max torque.
Mike

I think it clearly illustrates the negative effect of pushing a motor beyond what it is designed to do. More speed was available but at 50% more boost and therefore more fuel. It also generated a lot more heat which will lead to a short life. Heat is something which 6.0L owners seem to be concerned about.
Bottom line, you can't just go charging up every hill without a care, thinking everything will continue as it always has. One day you'll pay the price.


My take on Ford's programming is exactly the opposite to what you've found. Different motor of course, but the 5.4 just seems to settle in right around peak torque unless I force a downshift into first and generate a lot more heat.

[carringb]

Originally Posted by Silver350

My take on Ford's programming is exactly the opposite to what you've found. Different motor of course, but the 5.4 just seems to settle in right around peak torque unless I force a downshift into first and generate a lot more heat.

I usually just put my foot to the floors and let it settle in at 5-5200 RPM when I'm towing. It sounds nice.

[86Scotty]

Originally Posted by carringb

I usually just put my foot to the floors and let it settle in at 5-5200 RPM when I'm towing. It sounds nice.

Yeah, but you jump your van on gravel roads too! I just can't see doing that to mine, though I would agree that 5200 in a V10 probably sounds really good.

[Silver350]

Originally Posted by carringb

I usually just put my foot to the floors and let it settle in at 5-5200 RPM when I'm towing. It sounds nice.

Exactly. At about 14,000# , floored, towing a trailer, that 5.4 settles in at 2500RPM. On an 8% grade anyways.

The 6.8 makes peak torque around 4500?