Fuel Saving Thread: Motoring Tips & Tricks

[brewnog]

I honestly can not see how adding water to fuel can do anything but reduce the deliverable power to the engine. The main thing that water is going to do is draw off some of the energy to vaporize. That energy is unusable for the cycle and goes right out the tail pipe. That exact reason is why we (in aerospace engines) have to have a higher heating value and a lower heating value when dealing with engine performance calculations. I can see how it can help emissions, especially in the realm of NOx formation.

Indeed. Water, when added to fuel, just serves to dilute combustible product, decreasing the calorific value of the mixture in question. (Water, when used in a Diesel engine, will also quickly damage fuel injection equipment.)

 

[mtworkowski@o]

Was it true that during WWll, Germany had planes that used water injection? I'm thinking high compression, low octane and water as a means of stopping preignition. That's not the story I heard, but you seem to think that any water is bad.

 

[brewnog]

Was it true that during WWll, Germany had planes that used water injection? I'm thinking high compression, low octane and water as a means of stopping preignition. That's not the story I heard, but you seem to think that any water is bad.

Read my post (#89) again.

Yes, water injection was used in WWII.
Yes, it can be very successful for increasing charge density, lowering PCT, decreasing NOx, and widening detonation margin.
No, it's not the same as adding water directly to the fuel.

 

[mtworkowski@o]

I'm pretty sure I know that water injection is not the same as mixing water in the fuel. Still, what was the reason for injection and what exactly is charge density? Thanks

 

[FredGarvin]

You're right. They are not the same thing. The water droplets added to the inlet air evaporate and thus draw the heat of evaporation from the inlet air, cooling it and increasing its density.

 

[brewnog]

I'm pretty sure I know that water injection is not the same as mixing water in the fuel. Still, what was the reason for injection and what exactly is charge density? Thanks

Injection gives a fine mist of water which evaporates in the inlet tract. Water takes energy (heat) from its surroundings to evaporate, so the temperature drops. As temperature drops, volume decreases, so more mass per litre of air is admitted into the cylinder. More fuel can then be used, so more power is developed when the fuel is burnt.

 

[brewnog]

You're right. They are not the same thing. The water droplets added to the inlet air evaporate and thus draw the heat of evaporation from the inlet air, cooling it and increasing its density.

What he said.

 

[mtworkowski@o]

yep, I totally missed that post. thanks

 

[FredGarvin]

What he said.

No. What HE said.

 

[starkind]

pulse driving

I read about a driving technique called pulse driving reported to get more than 100 mpg in an unmodified factory model car. The idea is to accellerate to highway speed, then turn off the engine and coast down to about 30 mph. Restart the engine and repeat. Any physical truth to this?

Here's a link...

http://www.metrompg.com/posts/pulse-and-glide.htm

 

[turbo]

I read about a driving technique called pulse driving reported to get more than 100 mpg in an unmodified factory model car. The idea is to accellerate to highway speed, then turn off the engine and coast down to about 30 mph. Restart the engine and repeat. Any physical truth to this?

That would be extremely unsafe. People traveling at much above or below highway speed create higher relative velocities between themselves and other vehicles, which would result in a lot of fatalities. As a motorcyclist, I have found that the ability to accelerate out of a tight spot is just as valuable as having the ability to brake and avoid the tight spot.

 

[mtworkowski@o]

Let's think about how many times we have to accelerateagain. Acceleration is the biggest fuel eater. Always going to be better using a light foot and easy brakes. Why easy brakes you say? because if you don't make hard stops it means your off the gas early. I was born at night but not last night. Good question.

 

[mgb_phys]

The idea is to accellerate to highway speed, then turn off the engine and coast down to about 30 mph. Restart the engine and repeat. Any physical truth to this?

Apart from turbo's comments if you turn off the engine in a modern car you lose a LOT of braking and steering performance. You also have to be careful that you don't put the steering lock on!

A friend of mine did that back when I was an astronomer.
The road from the telescope down to the town was about 30miles all downhill with lots of switchbacks and pretty empty.
If you timed it right you could coast all the way without using the engine.
My friend decided to go one better and turn off the engine - unfortunately that engaged the steering lock and once he turned it full-lock in the first corner he found he couldn't straighten it.

 

[mtworkowski@o]

Does telling him the dangers of coasting with the motor off really answer his question? In terms of energy expended there is a real answer.

 

[mtworkowski@o]

What's the answer?

 

[mgb_phys]

I thought it was clear from the link he posted.
Pulse and glide is a standard technique for hypermilage, just not very appropriate for Highway use.

 

[starkind]

Actually I wasn't happy with the link I posted. No calculation, just handwaving. My first thought was the same as mtworkowski@o, that the cost of accelleration would surely be more than the benefit of gliding. But the thing seems to work, according to the postings. So, why?

And, how to calculate the savings from different max and min speeds?

I have actually done a little experiment, and satisfied myself crudely that there does seem to be an advantage. Also, I found that the other drivers did not seem to notice my speeding up or slowing down. This was on a 4 lane in low traffic, so they just changed lanes and passed. No one seemed irritated at all.

thanks. I'll probably think about it some more. Formulas, I want formulas.

R

 

[mtworkowski@o]

No, what's your answer? The coasting is only marginally better than the cruising. Cruising requires you to overcome all the resistances(rolling, air,). Those are still present but air is down during slow part of the coast down. Now for the next acceleration. This is like asking if it's better to feather the throttle or go as fast as you can and then coast. If you were running out of gas which would pick?

 

[starkind]

I should think to feather, being as going faster than 50 mph or so cuts the gas mileage due to wind resistance.

So if gas mileage at the steady speed is greatest near fifty mph, I could choose to experiment with upper and lower bounds at perhaps sixty and forty, so as to stay near the speed of greatest gas mileage.

The technique has two main phases...acceleration and gliding. I use acceleration here in the sense of increasing velocity. The gliding phase would be 'decelerating' due to radiation of heat energy through friction.

So there is a short burst of acceleration followed by a long period of radiation and slowing.

My eyes are blearing or my glasses are dirty. I'll sleep on it and have clean ones in the morning.

R

 

[brewnog]

The acceleration phase needn't be full throttle.

 

[starkind]

That's true. Which brings up the question of how much acceleration is most efficient.

I guess the way to a formula would be to start by identifying all the variables and setting controls. For example road surface and inclination would be variables in driving conditions, both of which could affect efficiency, but these are not part of the question and so should be held constant, by using a flat test bed with consistant surface.

mass of vehical
upper speed bound
lower speed bound
time required to accelerate
friction effects...tires, bearings, wind resistance, weather conditions.

I'm going to refresh my memory on the Carnot cycle.

 

[mtworkowski@o]

Don't bother. No acceleration is the most efficient acceleration rate. F=MA. No acceleration, no force. No force, no gas. elegant ay? This than requires us to supply just enough power to keep the car going at a constant speed. Overcome wind, rotaional, etc resistance. Now there is one more consideration. We know that air resistance increases with increase speed, so we don't want to go fast. But considering the engine power curve and the trans gearing, what is the speed that will give us the best gas milage?
Do you see how far away from your original thinking we've gotten? Have light bulbs been turning on in your head? I hope so. Learning is an amazing feeling. I should call it understanding. It's almost creepy how you "get it".
Let me know!

 

[starkind]

Hmmm.

Consider two otherwise equivalent cars at full stop. One starts out and accelerates slowly until it reaches highway speed one minute later. The other floors the pedal and (no skid marks) reaches highway speed in ten seconds. They both drive to the end of a sufficiently long course, and then stop. Do they both use the same amount of gas?

Moving the same mass the same distance means the same amount of work?

Then the best move would be to accelerate to speed in as short a time as possible, keeping in mind safety.

 

[turbo]

Hmmm.

Consider two otherwise equivalent cars at full stop. One starts out and accelerates slowly until it reaches highway speed one minute later. The other floors the pedal and (no skid marks) reaches highway speed in ten seconds. They both drive to the end of a sufficiently long course, and then stop. Do they both use the same amount of gas?

Moving the same mass the same distance means the same amount of work?

Then the best move would be to accelerate to speed in as short a time as possible, keeping in mind safety.

You have neglected mechanical efficiency.

 

[mtworkowski@o]

listen. you burn more gas accelerating quickly. F=MA The motor developes more hp and moves the car faster over time. sheeeesh

 

[DrClapeyron]

Open throttle gasoline engines, other than pre-ignition problems, what is holding it back?

 

[turbo]

Introducing more fuel and/or air to an ICE than it is designed for takes it very quickly out of the efficiency-range that it was designed for. Also, pre-ignition is a complex problem based on compression ratios, lean/rich ratios, and fuel grades. Are you sure you'd like to keep pushing on these subjects?

 

[mtworkowski@o]

cruise controll

verdict? Good for fuel or not? I know the answer.

 

[brewnog]

Open throttle gasoline engines, other than pre-ignition problems, what is holding it back?

Detonation, aspiration, heat rejection, scavenging, mechanical and thermal loading, control, fuel supply...

 

[OmCheeto]

hmmm... sorry I'm a bit late joining this foray.

I've been actively researching this topic for the last couple of years, and I discovered the best source of information is found by perusing the hyper-mileage forums.

There was one recurring theme that stood out: mpg gauges.

Automotive computers can do a lot to improve fuel savings, but the human mind seeing 3 mpg on the dashboard can do a lot more to change peoples habits than anything else.

Call your next president/prime minister and make it clear that this should not be an option in non-human powered vehicles.

And pump up those tires!