Going up steep hills- downshift or floor it?

[Tesla]

Diesel Vacuum,
Just thought I'd add, I think the general theory is no vacuum, that would be true at idle, but as air volume goes up, with the restriction of the air filter particularly, there is vacuum in the intake system, that is why they benefit so much from ram air setup.
I discoverd this when I set up a manometer on intake system, and am currently organising a vacuum gauge to set up permanently, this will give me a guide on how efficiently the engine is running using the info from both the vacuum & boost gauges, basically telling me the pressure difference across the turbo, less difference, les power drain by turbo.

[jtbo]

Originally Posted by 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.

Can't remember where it was, but I did read scientific document about different emission control systems and their effect to fuel consumption, I belive that link to document I found from this board, some topic, but anyway there was tested that EGR actually does increase fuel consumption, but there was other method including nitrogen that did decrease fuel consumption. It was within a week that I did read it but as my memory is not so great I can't remember where it was.

Name was emission controls effect to fuel consumption or something similar in meaning.

Does EGR do that in every case is then of course different question, as always to know how largely we can take finding as true, we need to know some background and testing methods, it can apply only to very specific application, where it certainly is true, but with some other application something else might be true. Especially with small amount of samples there can be always quite different results with applications not tested.

But I'm sure Tesla knows this very well, just heads up to anyone looking scientific tests that even they are not absolute truth that covers everything.

[jtbo]

Originally Posted by Tesla

Diesel Vacuum,
Just thought I'd add, I think the general theory is no vacuum, that would be true at idle, but as air volume goes up, with the restriction of the air filter particularly, there is vacuum in the intake system, that is why they benefit so much from ram air setup.
I discoverd this when I set up a manometer on intake system, and am currently organising a vacuum gauge to set up permanently, this will give me a guide on how efficiently the engine is running using the info from both the vacuum & boost gauges, basically telling me the pressure difference across the turbo, less difference, les power drain by turbo.

This depends from various of factors, airbox design, intake sound muffler design, what kind of air filter there is (there can be even more), I believe that there can be even restrictions between filter and cylinder, even intake valve can cause some restriction so it depends from engine design too with NA diesel engines, turbo of course does overcome restrictions but they still are there.

I think that vacuum gauge setup is good idea to tell if there is restrictions in air intake, but I don't quite see how you could measure power drain of turbo as it is propelled by exhaust and is creating 1 athmosphere even at very gentle driving.

At idle difference would be probably large as there is not much exhaust to propel the turbo and air filter restrictions would make less measurable vacuum, so pressure at intake manifold might be slightly less than what there is after air box. Mostly it would measure just effectiveness of ram air setup over different speeds, but it can be that I have missed something?

[Tesla]

This is the way I understand the whole turbo diesel thing,
Exhaust comes out of cylinder at high velocity, hits turbine and spins it, loses some velocity & heat, the turbine spins the compressor wheel which draws air through the intake and compresses it to go to the manifold and cylinders.
If you increase pressure(reduce vacuum) before turbo, it spools up quicker and requires less power to compress intake air, hence less exhaust pressure drop across turbine and exhaust travels out faster.
I imagine this then reduces pumping losses from the engine, the engine is pumping exhaust out if you restrict this with a turbo, it needs to pump harder. So the higher the pressure is before turbo, i.e. low vacuum, the less power drain there is from the turbo.
This can also be improved with a larger exhaust, but there is a lot more science in designing an appropriately sized exhaust.
Polishing manifolds and improving flow through the engine is really no different to the aerodynamics you do on the body, I think of it as engine aerodynamics, if the air flows better through the engine, then this reduces pumping losses, hence reducing overall fuel consumption.

[Tesla]

For the EGR thing I hate it and see it as a "necessary evil", or maybe if engines were tuned better it is an "unnecessary evil". Basically they are bad for engine longevity, where they may actually reduce fuel consumption is in the pumping losses, so if you connect the exhaust direct to the intake, as one cylinder is pushing out exhaust, another is drawing it in, like a closed loop, so minimal power is required in the transfer, but other factors come into it in the combustion cycle as to combustion efficiency. I don't know if it does improve economy, just read it somewhere a while back, I suppose my point was more that the ECU is doing a lot of things behind your back and what seems like a good move for economy may not always work out that way.
I got into this aero stuff after doing a lot of research on airflow through the engine bay for heat removal, and found both by reading & personal experience, what seems logical, is not always so and often you get a negative result from modifications if they aren't properly researched & tested.

[serialk11r]

Disclaimer: I don't have engineering degree, don't have experience with working on cars, don't have much experience with cars period, so this is all based off theory and info that I read

EGR is primarily for emissions reduction since it both lowers burn temperature by decreasing oxygen concentration and allows the unburned hydrocarbons to pass through the engine a second time. It has the benefit of reducing throttling by a bit, but I think emissions compliance is the main reason it's there.

Lean burn at low specific output is a better strategy as the greater amount of oxygen and lower intake temperature help efficiency, but for effective "throttling" based purely on reducing the amount of fuel, the air to fuel ratio is going to be very high and efficiency will be hurt, but since everything is happening at lower temperature theoretically it's better than EGR.

Ideally you'd want to be able to limit intake volume/pressure losslessly, and then increase the compression ratio as much as possible dynamically to keep the gas cycle efficiency up. However in this scenario, you still have to fight friction so at the end of the day BSFC still increases at lower load.

Tesla, as for pulse and glide on flat roads, you definitely are increasing energy spent to travel each distance because your speed is wavering. The average speed is a harmonic mean in some sense, so if higher speeds consume anything more than proportionately (linearly) more power, then not keeping a constant speed will result in a net increase in energy dissipated by the car.
The problem is if you look up a BSFC chart and figure out the power you need to cruise on the highway at say 60mph, most cars are operating their engines at 70% or worse efficiency, so pulse and glide easily makes up for the apparent disadvantage in power requirement.

[Tesla]

serialk11r
Not many of us here are experts here, we're all just trying to do the best we can.
Yeah, I know the EGR is just an emission thing, misguided in my opinion, and I'm not even sure about the economy thing.
As for the Pulse & Glide as said I definately practice this in undulating country, I see every hill as an opportunity and drag out the downhill stretch as long as possible, given favourable traffic conditions, but I am still getting my head around the whole BSFC thing and try to understand how it applies to my vehicle, as I get the aero bits better, then even smaller hills give good opportunities.
I might need to look at getting a duty meter on the injector signal, if I know what the injectors are doing, then that may be a way of evaluating the pulse & glide concept on the flat.
All good food for thought.

[serialk11r]

I think it's probably more effective on cars with shorter gearing since they are the ones that run less efficiently at steady state. I haven't ever tried it since I don't have my own car, and my mom would freak out if I were putting the car in neutral on the highway, but I would be a little scared to do it with other cars around because I feel like a lot of drivers use either cruise control or just go as fast as possible as long as it's under 80mph . Making the pulse intervals short would require more work and possibly more drivetrain wear, making the pulse intervals longer would confuse other drivers and possibly create a dangerous situation.

The theoretical gains possible are really attractive though, I will probably end up trying it someday.

[jtbo]

I think that in regards of hill climbing, BSFC could only tell possible problem areas, like very low throttle around 2000rpm which in some vehicles can mean over twice of consumption compared to lower rpm and higher load, but as it is engine specific one really would need map for his vehicle, which in case of my car with XUD9TE engine is next to impossible to find I have found out.

But, one has to remember always that it is per power unit, so sometimes being in bad BSFC area is not so bad as amount of power requested is so low.

When accelerating power request is for very short time, so it can be better to use that 70% at max torque rpm, but just briefly.

When climbing up a hill one has to work out time, how long going to need power, usually optimal is to let speed decrease so that at peak speed is low and hill builds up speed, even for longer hills, but with longer hills one has to consider how long and what amount of power is needed, then which gear and speed will give less consumption and there BSFC table could be meaningful to calculate different total consumptions.

So BSFC in my opinion is not all the answers, it is completely useless for many situations, but it can be useful to avoid some pitfalls with long hills.

Still it is learning process for me, to understand all things that affect to end result so that I would be able to use all that information to make good decisions.

[redpoint5]

I haven't seen anyone directly address some of these misconceptions, but Old Mech's post was very accurate and informative. Here goes...

Originally Posted by Mulder25

Anyways, the more you press the pedal, the higher your GPH will go, regardless of engine RPM, it increases the amount of fuel injected into the cylinder. At no point, can 80% of your max fuel rate be better than 20%... i just cannot imagine that 80% throttle in high gear would be better than 20% throttle in a lower gear.

You would think that less throttle position would also mean less fuel consumption, but that's only half the story! Most people imagine the throttle as a valve on a water facet, with a constant flow that is dependent upon how open the valve is. This isn't the full story though. Engines don't flow a constant stream of fuel, instead they consume pulses of fuel during each injection event.

The throttle is a valve, but it controls air flow. When the throttle is opened, more air is allowed in the cylinder and proportionally more fuel is injected. So throttle position addresses the question of how much air (and fuel), but it doesn't address the question of how often. Fuel must be injected into each cylinder every 2 revolutions of the crank on a 4-stroke engine. At 2000rpm each cylinder is requiring 1000 fuel injections per minute. At 4000rpm, each cylinder is requiring 2000 fuel injections per minute; that's double the amount of injection events!

While dropping down a gear may allow a lower throttle position and less fuel injected into the cylinder, it is increasing the rpm and requiring that lower amount of fuel more frequently. As others have pointed out, more rpm means more energy lost due to friction as these parts move faster (further).

It is more efficient to have big explosion events less frequently, than to have smaller explosion events more frequently. Remember that in a 4-stroke engine, only 1 of those strokes produces power. Double the engine speed and you are doubling the amount of intake, compression, and exhaust events; all of which produce no power but instead waste power due to friction.

Originally Posted by Mulder25

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.

By now you should know that keeping RPM low but throttle high is more efficient.

Originally Posted by Tesla

...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.

You have probably already arrived at the answer to this. The engine "breaths" best when the throttle is most open.

Originally Posted by Tesla

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.

All correct except the last part. Result is higher economy.