dsmdriver

iTrader: (3)

One of the primary reasons that deisel engines get much better fuel economy is that they don't have a throttle plate- they modulate power by modulating fuel delivery.

The fact that you can run near WOT in an airplane all the time and modulate power with altitude also means that their engines can be more efficent overall. My dad gets 25 MPG at 200 MPH in his airplane. Pumping losses are pretty important, and if you can overcome them by getting rid of the throttle plate you can see pretty big efficency gains.

In a gasoline engine with a throttle plate, MAP (manifold absolute pressure) and RPM are what determne power (assuming you are running a standard mixture). You can't boost the pressure in the manifold without increasing HP, which requires more fuel. If you pressurize the area before the throttle plate, you have to close the throttle a bit in order to maintain the same HP. This actually increases overall pumping losses.

Turbos can get higher overall efficency because they recover the energy from the exhaust stream. But something that steals power from the crank won't have this luxury. It seems to me that a blower that pressurizes all the time will have a double hit- the pumping losses will increase, and the blower steals this energy from the crank. I believe this is why lots of street blowers have bypass valves - to prevent a MPG hit when crusing on the highway.

neit_jnf

the primary reason for diesel engine's higher overall efficiency is the high static compression ratio, throttle plate and pumping losses are also important

Richard Paul

iTrader: (5)

Very good until you get to the last paragraph.

Turbos can get higher overall efficency because they recover the energy from the exhaust stream. But something that steals power from the crank won't have this luxury. It seems to me that a blower that pressurizes all the time will have a double hit- the pumping losses will increase, and the blower steals this energy from the crank. I believe this is why lots of street blowers have bypass valves - to prevent a MPG hit when crusing on the highway.[/QUOTE]



Turbos don't get better efficency, period. Now lets see why, "free power from the exhaust" false. I want you to put an orfice in your butt and then tell me it doesn't take more power to discharge your waste. There is always an orfice in the exhaust on a turbo car. If you think a little restriction doesn't hurt that much then why all those header sales. On top of that how come some with very little difference do better then others.

When you produce pressure in the exhaust system in order to make the compressor side work and make boost there is a byproduct of heat. This heat must go someplace, into the water then into the radiator. It goes out everywhere including the engine compartment. And everywhere else.

The turbo has the same losses from blowing through a TB. It is the same no matter what, they are both superchargers. Yes the blowoff valve helps but it is not the best way. As you stated.

So it seems now that it doesn't matter where you "steal" the power to drive from. A compressor take X power to run and it doesn't care where it comes from. Both ways are parasitic but they return more then they take.

A SC set up with the TB before itself is running in partial vacuum therefore they don't take much power. (drain from gears, bearings, belt drives, seals etc still drain though) In the case of my compressor when running at cruise takes so little that I can't calculate it. I also offer the theory that being a better pump may help the pumping loss.

All in all a good thought on your part. You can't be flamed for buying into the BS that turbos get to recover free drive power. It is not true. Consider the delta between the turbine and compressor. On long range ships they can get good results because they use large turbines. They have no response, it takes miles to get up to power. For lag reduction street turbos use small turbines ans small orfices. These cause backpressure all the time.


I'm tired of typing so I'll go now

Richard Paul

iTrader: (5)

BTW, tell us more about your dads magic airplane.
You might be off by 100% fuel used.
But then again there may be things I don't know about flying around.

Most likely he's telling your mom a fib so she thinks it is cheaper than it is.
I used to do that with our boat.

guitarjunkie28

damn we're gonna have fun tomorrow !

rotarygod

Are you 2 getting a room?

guitarjunkie28

why, you want in on this?

deppenma

get back to work

Richard Paul

iTrader: (5)

guitarjunkie28

good god, the old man's even dirtier than i am.
that's just scary

Richard Paul

iTrader: (5)

Dave, I don't have my cell phone until tomorrow sometime. We switched companys and now we're switching back. I'll give you a landline by PM.
Note that I usually don't answer it but I will for now.

Umbra

Hehe, I love all this hot air, I think we need more butt plugs.

Contant fuel with less air (hotter air) means less oxygen for combustion which mean less fuel burned. Theoretically of course. Generally adding water to the charge is a cheap way of adding oxygen in addition to cooling.

dsmdriver

iTrader: (3)

I thought we were having a discussion here, not an instant flame fest.

My understanding is that turbos get most of their power from heat recovery from the exhaust stream, not the flow. There is a lot of energy to be had in the heat that is in an exhaust stream, and if you measure the temperature before and after the turbo, you'll see that the turbo recovered some of this energy by dropping the temp of the exhaust. This is actually recovering some energy that would otherwise be lost, and helps the efficency of the turbo. It doesn't make the turbo perfect, but it does make it more efficent than a crank driven SC.

I agree that deisels have a lot of effiency things going for them as mentioned, but the lack of partial-throttle pumping losses is one of them for sure.

If you want to really step out of your element and riducle something you know nothing about, and then loook like a uninformed dolt, here's the info you want about small aircraft: My dad's airplane is a Van's Aircraft RV-6. It's a homebuilt plane, with over 3000 of them flying. It uses a Lycoming IO-360 engine, which is 180 HP at sea level. At 164 knots groudspeed @ 12,000 feet it burns about 7.5 GPH when leaned properly. That's 189 MPH, divided by 7.5, gives you 25.2 MPG.

There's nothing magic about it- small, light, well designed aircraft do this every day. If you doubt me, do some searches and you'll see my numbers are pretty common. I'm a pilot as well and spend my days designing engine instrumentation for small aircraft, so I know what performance you can expect. There are even more efficent planes than the RV's if all you are looking for is fuel economy.

In the end when we are talking abut pumping losses, we are discussing low throttle applications like highway cruising. From what I know, this is the situation where most superchargers use a bypass valve to increase economy, and this is because the SC will be pulling energy from the crank that isn't helping the engine at all. Please inform me if I am wrong about the purpose of the bypass valve.

dsmdriver

iTrader: (3)

Water injection does not add oxygen to the combustion cycle. If it did, the hydrogen would have to break off, and suddenly you'd have hydrogen and oxygen in seperate molecules. As we all know, hydrogen and oxygen together make a great flame. What's the byproduct of combustion? WATER.

If burning water made H and O2, then the H and O2 would burn and make water. You'd have a perpetual energy machine that would run away and burn up the world.

Water is the lower energy state of H and O2. You have to put energy into it in order to break it apart. If WI broke apart the water, you'd be loosing tons of energy in your engine while spraying.

The purpose of WI is to lower charge temps and reduce detonation. The water turns to steam which lowers the temp, and the water in the cylinder acts as a buffer between the hydrocarbon and oxygen molecules and slows down the flame front, helping to control detonation. Since you gave up some of your energy by helping the water gto to steam, you actually need to burn more fuel in order to get the same HP as before, but the point of WI is not effiency, it's max HP.

guitarjunkie28

but most importantly for the renesis guys, the water cleans off the rotor faces and prevents future detonation.

rotarygod

I have a little exercise for you. Put a turbo in an oven. Does it spin? Now blow cold air through it. Does it spin?

There is a lot of heat in the exhaust system however it isn't the heat that is doing the work on a turbo. It's the flow. Heat plays an important part though in that it affects turbo exhaust wheel/housing sizing. This is because hotter air takes up more space than colder, denser air. The hotter the air is, the larger the turbo's hotside needs to be to flow it efficiently. This is why the rear mounted turbos need smaller exhaust wheels. The exhaust gasses are cooler and take up less space. Even though the turbo is sized differently, based on the temperature and energy present in the exhaust gasses, they are the same restriction to the system from a functional standpoint.

Yes it is true that the turbo helps remove some heat from the exhaust system. It gets absorbed by it and transferred into the oiling system, housings, and in some turbos back into the cars cooling system as not all turbos have coolant lines on them. It also radiates this heat back into the engine bay. None of it gets used to make power. Only the flow does that.

Your dad's plane sounds pretty cool. There are many rotary powered RV8's flying around and yes I agree that they do get the mileage you say they do. For a small plane, they are pretty damn fast. Light and powerful is a good combo. That plane will make the average Cessna look like a truck in the performance department.

rotarygod

Water injection is not going to add any appreciable power making oxygen into the system. Think about it for a minute. How much air does and engine need at even idle speeds? How large is the total volume of the water tank and how much of it gets injected per amount of time? How much of that water is just oxygen? I hope no one thinks that is enough to make a difference. Especially not at higher rpm's when under boost. There isn't even enough there for a human to take more than a single breath or so off of the whole oxygen content of the tank. Remember, it's only a third of the total amount of water.

SC-ed

Yes, you are wrong. It is not about economy. The SC is building pressure, which is not needed at cruising speeds (no engine load). The bypass valve keeps that intake pressure low. Think of it as a blow-off valve (kind of).

rotarygod

Any form of forced induction takes power to make it. It has to. When it's not making power, they all still take some power but not much. This applies to turbos and superchargers. A bypass valve is commonly used with positive displacement superchargers. They make boost according to rpm which means even at cruising speeds at low load levels. Remember it takes power to make it so if they are boosting, they are robbing the engine of power. It only takes so much to keep the car moving. If you have boost at the same rpm, you have more air which is more fuel and less economy. By using a bypass valve, the boost pressure is bled off to the frontside of the blower so in essence the only power being robbed is the power to keep the blower free spinning. This isn't much power. In contrast, on a turbo system the lack of a good scavenging header combined with an exhaust restriction will hurt economy more than on a supercharger.

Now you are probably wondering why you need a bypass valve if the blower is after the throttleplate spinning in what is in essence a vacuum. We are never in a complete vacuum. Even with the throttle plate completely closed at idle, you still have 20% of atmospheric pressure in the system. As long as there is air, it can be compressed. If you have a positive displacement supercharger remember you are theoretically able to make full boost at cruising speeds. With the throttle plate cracked open a little bit for cruising speeds you are still at about 40% of ambient outside pressure. This means the air still gets compressed by the blower by a similar amount over what it could do with the throttle wide open. If your blower makes 8 psi at full throttle at this rpm, it will still make a couple of psi at cruise. Even if it didn't make positive pressure, the pressure in the manifold is still greater than it would be without the blower. What is boost? It's pressure greater than what ou would normally have. It doesn't necessarily have to be positive. Just greater than what was there. More air needs more fuel and takes more power to create in the case of a blower. This takes power and is more air than the engine needs. Install a bypass valve and the engine no longer is working to compress the air. Now it is free spinning and using less power.

Why wouldn't the axial flow, centrifugal superchargers, or turbos need bypass valves though? Could they use them? Sure they could. Would it help appreciably? Probably not. Why? Remember the boost profile of each type of compressor. A turbo is well out of it's efficiency range at cruise (depending on it's size but I'm not going to get into that here) and is more or less supplying the air with only what it needs due. A centrifugal isn't really making much boost here anyways and when you cut that number down to account for the pressure in the manifold, it results in practically no extra air. In some cases the centrifugal hurts airflow at lower rpm's. The axial flow is similar in that it isn't producing much boost at lower rpms although it should be more than a centrifugal. The axial flow doesn't take much power to run anyways and doesn't heat up the air much compared to other forms. Combine this with how little air pressure is in the manifold and it's boost profile and you'll see that a bypass valve may be more effort than it's worth.

One thing to keep in mind is that a bypass valve actually causes a supercharger system to "lag". On my friend's old Bonneville SSEi, when you stepped on the gas, you could tell there was a slight lag until the bypass valve closed. It was a lot like turbo lag.

dtorre

Yikes lets stop with all this debating....I feel like im back in class with RG's posts >< ......Lets keep it to updates from now shall we =)

dsmdriver

iTrader: (3)

Actually, a normal breath is .5L of air. Air is 20% oxygen, but in the vapor phase. You only need about .1L of oxygen per breath, and you actually only use about 2/3 of this. The rest is wasted and never makes it into your lungs.

Liquid water expands about 1600 times larger when it goes to vapor.

So a 2L water injection tank would be about 3200L in the vapor pahse. This would be a 33% oxygen mix if you broke it up, so you could take about 5300 .5L breaths off of the oxygen in a 2L tank of water. At a normal rate of 20 breaths per minute, this would last you almost 4.5 hours. This all assumes perfect effency of delivery, etc, but you can cleary take more than one breath off the O2 in a tank of water.

Liquids are really dense in comparison to vapor.

P.S. May be off by a factor of two- need to think about the fact that you need O2 and H2O breaking up gives you a single O. You should really do this math based on mass...

dsmdriver

iTrader: (3)

"Not needed" - who cares if it isn't needed? You only deal with "not needed" stuff if they are doing harm. The harm in this case is the wasted energy that hurts MPG.

A blow off-valve is there because turbos can overpressure when you close the throttle and this hurts the turbo bearings, the throttle plate bushings, and slows the turbo, creating more lag. Crank driven SC's don't have overpressure issues because they don't have any inertia like a turbo does- they always follow the crank speed.

Richard Paul

iTrader: (5)

I wasn't flaming you in fact I said a couple of nice things.

When it comes to the airplane, my bad, I didn't think homebuilts. My thought when you said 200 MPH it must be a Moody or a Saratoga with a 300hp in it. Therefore I was thinking 13 GPH. So ban me.

All of it sounds like great milage to me. I'm used to a Merlin.
I no longer fly since I lost three of my best freinds, great pilots all. Just as Reno racers they liked flying close to the ground. Always feels faster that way, doesn't it. In Zeuschel's case it wasn't that, it was a flame out in an F-86.

It's not out of my element at all, I'm just a little dated. Maybe jaded. Homebuilt to me means a P-51 with short wings and a tiny canopy. In one case even the underscoop and radiator removed and put in the wings. How about runing them with 50psi boost? And 3750 RPM. Rolls said GOOD LUCK! Mileage was never an option.

To bad I pissed you off, I could use a couple of gauges that aren't off the shelf.
Like an intake manifold temp with high speed sender. A dual needle pod with the pressure on the other side. All in a 2.25 gauge, or 3.125 would be even nicer.

caribbean_spice_boy_73

I hear you.. but I personaly look foward to reading his post.... a little spice from RG & RP to keep it right...

dsmdriver

iTrader: (3)

I'm not as far off from understanding you as you think I am RG.

I wasn't trying to claim some sort of super-effiency in a turbo or anything. I was just saying that I believe, in highway cruising, that they have a slight MPG advantage over any type of crank driven SC.

I am fully aware that anything that pressurizes a system requires energy, and some of that energy will be lost, and where the pressure increase in the system occurs isn't relevant. It appears to me that anything that increases the pressure in the system (before or after the throttle plate) while the power is not needed is bad for MPG. For best efficency, you only want to start boosting once the throttle is at 100%. This obvioulsy doesn't work for throttle response though.

And more fuel and more air means more HP, which means the car accelerates. The driver then backs off the throttle until the right amount of HP is achived to maintain speed. The more closed throttle position means pumping losses have increased though, since there is a higher pressure drop across the throttle plate. This is why any unwanted boost is always bad for economy.