Going up steep hills- downshift or floor it?

[trooper Tdiesel]

yes, less throttle the higher the vac is.
more throttle lower vac is.



i use a boost gage on my diesel, for the same purpose a vac gage is used on a gasser. it also shows vac on it, so i know when my air filter is getting dirty.
not exactingly the same thing since i have just about 0 boost under 2,200 rpms.
being a VERY near 100% computer free auto, (one small circuit board for the QOS 5V type glow plugs) there's really no other option that i know of.

well OK a EGT gauge would work too, but have a bit of a delay.

[euromodder]

Originally Posted by Mulder25

I always try to keep my RPMs low, and my throttle position low.

While you're likely getting the best fuel mileage that way - low rpm, probably low(ish) steady speed - you're not getting the most bang for your gallon of fuel.

In those conditions, your engine is burning its fuel less efficiently in terms of HP / gallon.
But because these cruising conditions require so little HP, you still get good fuel mileage.

It feels contradictory, but it isn't

[jtbo]

Originally Posted by euromodder

While you're likely getting the best fuel mileage that way - low rpm, probably low(ish) steady speed - you're not getting the most bang for your gallon of fuel.

In those conditions, your engine is burning its fuel less efficiently in terms of HP / gallon.
But because these cruising conditions require so little HP, you still get good fuel mileage.

It feels contradictory, but it isn't

I believe that is nicely put what is challenge of trying to figure out what is way of getting lowest fuel consumption, even with hill climbing going with lowest HP that one can is best, it is just that below certain rpm lower gear can provide that low hp figure from much more favorable spot from BSFC table, which is then reason to downshift, then only challenge that remains is to know what rpm is too low.

Lugging engine at uphill is probably too low

My speed goes down to 50kph on long hills, my normal driving is around 80kph, I think 50kph was below 1400rpm and there is not much of any power left, throttle would need to be nearly floored to maintain the speed and engine is really not liking to go.

Wheel power might be quite low on that situation already.

Two choices for situation, downshift and keep hp low or originally at down of the hill push more throttle and request quite bit more hp to maintain speed which you can stay at top gear.

In such situation downshifting is better as requested hp will be lot less, even it will be less efficient and even there will be more losses because of high rpm.

Luckily there is only few hills that I need to do that, rest are those low throttle, low rpm on top gear type of hills, but that shows how one have to adapt to hills, also one need to plan ahead, see what is other side of hill, is it flat or is there downhill and plan accordingly, if there is downhill one can let speed decrease lot more as downhill will increase speed again with neutral coasting, if there is level ground, then it would be accelerating with engine, where one has to figure out if it is better to ask more hp at hill or at level ground?

That is why trying to get lower consumption is also more interesting at hilly ground as it is far more challenging, requiring a lot of planning.

[user removed]

Originally Posted by Tesla

So what you are saying is basically the same as I wrote a couple of posts up, although mine is a Diesel, this is what I have generally done with most vehicles I drive.
I have a question though, how does this relate to the concept of using a vacuum gauge on gas engines as an economy gauge, i.e. higher vacuum, better economy?
Surely running in a higher gear with low engine speed and a more open throttle would result in lower vacuum & hence lower economy, only one concept can be right.

In a throttled IC engine a higher gear and lower RPM-higher load will give you better mileage at steady speeds.
To better understand load, think of driving with the throttle locked in a position, say 25%. You start climbing a hill that gets steeper the more you climb. Now you are using fuel to maintain your speed as well as increase your altitude. In essence you are lifting the car higher using the hill as an inclined plane.
As the hill's grade increases your car has to work harder to maintain it's speed, and you have to increase the throttle position to keep it going at the same speed. This is due to increased load from lifting the weight of your car and it can be calculated precisely using the formula for horsepower.
At some point as the grade keeps increasing you can no longer maintain the same speed in the highest gear. This is the maximum load the engine can produce at that RPM.
In order to maintain the speed you will now have to downshift to a lower gear and increase your RPM to maintain the same speed. As the percentage of the grade increases you will have to again increase the throttle position until it is at maximum, then downshift to an even lower gear to maintain the speed.
Load is not directly related to throttle position, but in many cases you will have to increase the throttle position to maintain speed uphill because the work required to climb the grade is added to the work required to maintain a constant speed. In a way a hill allows you to store energy in the mass of your vehicle which you can use on the downhill portion of your trip, in some cases you can even cut the engine off and maintain the same speed without any fuel being consumed.
Driving With Load means you are trying to stay at the same vacuum reading regardless of the changes in elevation. It's a good strategy in hills as long as they are not too steep. Another strategy is to increase your speed on the downhill portion and let it drop off on the uphill portion (like an unpowered roller coaster).

When your engine vacuum is high, efficiency is low, even though your mileage may be it's best. This is because high vacuum means low compression, which means less work for the same amount of fuel consumed.

A 4 cylinder engine producing 20 HP at 2000 RPM uses X amount of fuel. If you increase the load so the same engine is running at the same speed but producing 50 HP, the fuel consumption will be 1.5X. Basically the extra 30 HP only cost you half again as much fuel.

At the 20 HP load you are using .5X fuel per 10 HP.
At the 50 HP load you are using .3X fuel per 10 HP.

This is why pulse and glide works well to increase your mileage. It's almost the same as climbing a hill then coasting down the other side, except there is no hill. You are using the mass of the vehicle to store inertia and then using the inertia to travel with no fuel consumption (assuming you shut the engine off). Even if you let the engine idle, coasting at 60 MPH with an engine using .25 GPH fuel, you are getting 240 MPG at 60 MPH coasting.

regards
Mech

[jtbo]

Old Mechanic, that is very informative post there.

Now how it would be different if there would be no vacuum at all? I think that I'm right when I'm thinking that without vacuum higher rpm low throttle has much less issues, but is that really so?

[user removed]

Higher RPM
Low throttle
Without vacuum

Not sure how you would accomplish that combination, assuming I read the intent of your post correctly. High RPM and no vacuum can not really be accomplished with low throttle positions. Low vacuum is high load which means the engine is working harder. It's not really possible to have a high load and high RPM with a low percentage of throttle opening, unless you went from a high percentage of throttle opening to a low percentage, which would raise the vacuum reading for a short period of time until the inertia of the vehicle depleted and the load increased.

regards
Mech

[jtbo]

Originally Posted by Old Mechanic

Higher RPM
Low throttle
Without vacuum

Not sure how you would accomplish that combination, assuming I read the intent of your post correctly. High RPM and no vacuum can not really be accomplished with low throttle positions. Low vacuum is high load which means the engine is working harder. It's not really possible to have a high load and high RPM with a low percentage of throttle opening, unless you went from a high percentage of throttle opening to a low percentage, which would raise the vacuum reading for a short period of time until the inertia of the vehicle depleted and the load increased.

regards
Mech

I have no throttle butterfly, so no vacuum either, oddities of diesel

So I guess my initial thinking must be right that with diesel it might be better to use higher rpm instead of lower when getting up the hill, which makes driver need to adapt to car he is driving and that is good to know.


Here I present my shopping route, it is bit of challenging route for fuel economy, this is just 20 miles from shop to home, when I start driving engine is of course relatively cold, but that first hill (right in graph) will get engine up to temp quickly, however it is taking lot of fuel climbing that at warmup cycle.
My last fill up was 5.1l/100km (around 46mpg) and it had mostly driving that route, some short trips and coast down testing quite a bit, I can get bit better mileage on that route at summer.


Good thing is that to other direction I can get miles of engine off coasting as there is good combination of hills, almost from 577 to 401, there is need to use engine around 20 seconds at hill near 449, but only because there is 60kph limit and school without those I could coast whole way from 577 to 401. Each datapoint is between around 42 meters, which is almost 138 feet, so that is over 7km or rather would be. So certainly not the worst kind road, but it would be easy to burn a lot more fuel on these hills.

[serialk11r]

On a diesel the same principle applies to load, more load up to ~70% or something is increasing efficiency, after that adding more fuel and making more power decreases efficiency.

I think on a diesel the accelerator pedal directly controls injection duration but not sure.

[jtbo]

Originally Posted by serialk11r

On a diesel the same principle applies to load, more load up to ~70% or something is increasing efficiency, after that adding more fuel and making more power decreases efficiency.

I think on a diesel the accelerator pedal directly controls injection duration but not sure.

More modern might do even more than that, but amount of fuel that goes to cylinder.

I was wondering if adding potentiometer for throttle linkage and connecting that so that it would adjust voltage of voltage gauge, would make nice throttle position indicator. For gasoline burners vacuum gauge does that, but with diesel there is always more or less 0 vacuum or then some boost pressure, which I avoid mostly as motor is providing enough power without boost for slow hypermiling.

For me it is hard to know if I press 50% or 90% throttle, but I put sponge under throttle pedal so that it is really hard to floor it, works really well and it is not much more comfy too on long trips

For some reason when I tested top gear and more throttle I got boost at same hill where on 4th I did not get boost, I could keep lot less throttle, but I had to push pedal quite bit down to maintain correct amount of speed decrease.

Another gauge I miss is current output power as I could use that to optimize my power usage and thus consumption. It could be done by software though, addition of potentiometer on throttle linkage, rpm information and boost information should give enough data to choose amount of power from table, just making correct table is big challenge.

[Tesla]

I'm keenly reading all the input here, I have a science background so understand all the momentum, gravity, potential energy stuff.
Just trying to get my head around the whole pulse & glide thing, there is a few runs I do through undulating country, where I do the no brakes thing, all about timing the crest, to just roll over the top, allow gravity to pull you down, throw in a bit of juice at it on the next rise, then go again, this works well.
But I'm not convinced that doing the same on the flat will have the same benefit, particularly as vehicles have so many different systems adjusted by ECU that in some vehicles at least it would be more economical to hold speed constant at low load, eg many vehicles engage EGR at this point and I believe fuel consumption goes down at those times, but I don't know for sure, just bits I've read around the place.