Going up steep hills- downshift or floor it?

[Mulder25]

Originally Posted by serialk11r

Mulder25 are you talking about literal throttle position? Electronic throttle cars are probably doing worse at 20% than 80%, although that's 20% dictated load vs. 80% dictated load. 80% literal throttle position or 80% on a cable throttle would be almost 100% load at low rpm, and 20% throttle position would probably be landing you much higher in terms of load too.

I am talking about the scanguage reading. With the throttle out, mine is at 10tps. Floored it is at 90tps. So 80% would be 74tps. I do not understand the entire concept of engine load, i have tried doing research and this whole DWL thing just doesnt make sense, there is no good article out there explaining the physics behind it, and how to implement this hypermiling technique.
I did the test today, and came up with 1.28GPH average in third gear, and 1.14GPH average in fourth gear. This comepletely defies common logic, but you cannot argue with a scientific instrument, so i guess i will be launching from second, and driving in fifth at 30MPH now. It does make sense in the fact that you have less torque, so you arent wasting gas on quick accelleration, but i thought the less torque was due to inefficiencies in a higher gear at low speed, not a compensation for power/energy.

[Tesla]

Just went and did a bit of reading on all this BSFC stuff, as far as I can ascertain, it is the point the engine reaches it's highest efficiency, which generally is around the peak of it's torque curve with open throttle. Generally peak torque is significantly below peak power, so when driving at peak torque rpm will feel like the engine is labouring a bit, but just slightly over that rpm, it will generally have a nice kick if it isn't overloaded.
My engine reaches peak torque at around 2,000rpm & max power at 3,600rpm, from my experience the engines sweet spot is around 2,500, which equates to 100km/hr in top gear.
So from the bits I read, the indication is to try hold the engine rpm at around maximum torque on a mostly open throttle, not easy as load keeps changing.
Now if you had the engine mated to a suitable alternator/motor through a cvt
the engine could be set to run at ideal BSFC, low load it charges batteries, high load the electric motor does the boosting, but that's what Hybrids are all about, aren't they?

[serialk11r]

I think peak torque has a lot more to do with peak volumetric efficiency rather than peak efficiency period.

I'm not 100% sure on this but peak thermal efficiency rpm is mainly friction/cooling and sometimes combustion efficiency. Higher rpms always mean more friction, but higher rpms always mean less cooling loss because the burning charge has less time to cool as it expands. In addition, since the intake air is moving faster at higher rpm the charge is more homogeneous and burns better.

But by far the most important factor in BSFC is load, because decreasing load means you're spending more of your engine output to overcome friction and throttling losses.

Anyways Mulder25 the reason you have less torque in a higher gear is not because the torque available at a lower rpm is significantly lower, but because in a higher gear you have less torque multiplication to the wheels. I don't think starting in 2nd will save gas because if you start in first, you spend less time slipping the clutch, but shifting to 5th at 30mph is a good idea. As long as you're using the available power in a lower gear, it's okay to use that lower gear.

[trooper Tdiesel]

the closer i am to max torque rpms. 2,200 the more i can back-out of the go peddle.

but being a old school diesel with turbo, my boost falls on its face under 2,200 rpms and im quickly at 100% peddle and loosing speed.

so in my case, down shifting and holding rpms at 2,300/2,500 is best.

[IsaacMTSU]

My commute is pretty hilly, in my CRX I never press the pedal more than 75% so I try to keep it in 5th at 70% pedal position until my speed bleeds off to where I'm not making much torque and just shift down to 4th and sometimes down to 3rd. I don't have a gauge but it seems flooring it in 5th is worse than driving easy and downshifting. I never hold a steady state unless I bleed off too much speed and need to stop it from losing any more.

[Tesla]

It seems to be about matching the engine rpm to load, ie if you are going at slow speed on the flat and can accelerate slowly, then nstay in a higher gear, but if you are going into an uphill run, no point in staying in high gear open throttle, because the engine is not going fast enough to consume what you are giving it, better to change down and let the engine rev up and breath properly.

[Mulder25]

I also found this on a wiki article about variable displacement:
In typical light-load driving the driver uses only around 30 percent of an engine’s maximum power. In these conditions, the throttle valve is nearly closed, and the engine needs to work to draw air. This causes an inefficiency known as pumping loss. Some large capacity engines need to be throttled so much at light load that the cylinder pressure at top dead centre is approximately half that of a small 4-cylinder engine. Low cylinder pressure means low fuel efficiency.

I can't imagine that this would have a really significant effect, so much as friction inside the cylinders would. I always try to keep my RPMs low, and my throttle position low. I avoid higher than 3,000 RPM and try to keep my TPS less than 19(starts at 10, goes to 90) while cruising. Im going to do some significant research on engine load, and what exactly it means, but thanks to all who replied, it did help.

[serialk11r]

You're probably correct that it's not as big of an effect as the piston skirt/ring friction, but it costs a non-trivial amount of power.

You want to reduce throttle position if you're under light load, because that equates to less fuel burn, and a higher gear will almost always let you do that, the rare exception being when going up a steep hill or maybe with certain turbo engines. Remember, if the engine speed is lower, the volumetric efficiency at the same throttle position will be higher, because the pressure drop is lower across the same throttle plate if the air is moving by slower. However when you are accelerating, that is the perfect chance to open up the engine and operate it at higher efficiency levels, so you don't want to feather the gas while accelerating.

[jtbo]

Originally Posted by serialk11r

You're probably correct that it's not as big of an effect as the piston skirt/ring friction, but it costs a non-trivial amount of power.

You want to reduce throttle position if you're under light load, because that equates to less fuel burn, and a higher gear will almost always let you do that, the rare exception being when going up a steep hill or maybe with certain turbo engines. Remember, if the engine speed is lower, the volumetric efficiency at the same throttle position will be higher, because the pressure drop is lower across the same throttle plate if the air is moving by slower. However when you are accelerating, that is the perfect chance to open up the engine and operate it at higher efficiency levels, so you don't want to feather the gas while accelerating.

I have been wondering this a bit from point of diesel, as there is no throttle butterfly, it is always kind of WOT, but just lean mixture, which leads me to believe that even when accelerating one can get rather good results by accelerating relatively slowly as long as one is top gear.

[Tesla]

Originally Posted by serialk11r

Remember, if the engine speed is lower, the volumetric efficiency at the same throttle position will be higher, because the pressure drop is lower across the same throttle plate if the air is moving by slower. However when you are accelerating, that is the perfect chance to open up the engine and operate it at higher efficiency levels, so you don't want to feather the gas while accelerating.

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.