A discussion on engine efficiencies - Page 4 - PeachParts Mercedes-Benz Forum

t walgamuth · #46 08-28-2005, 07:31 PM

the pinto was a lot worse than most.

when rear ended, the body opened up and the rear axle split open the gas tank so that it all spilled inside the passenger compartment and toasted the occupants. an engineering study was done by ford to determine the cost of a repair. it was determined that a $.50 steel plate would reduce the danger dramatically. ford's genius accountants determined that paying off the families of fried people was cheaper than putting on the plates and so ford continued building them the same way. this all eventually came out in a law suit. and is a big reason why i like driving benzes.

tom w

Roncallo · #47 08-28-2005, 10:20 PM

Quote:

Originally Posted by Rick & Connie

I came to this site wanting to learn more on my MB.And hoping to help others with my mechanical experience.
This started on someone elses thread.And I'm hoping to clear up misunderstandings with this one.Especially if the misunderstandings are my own.So here goes.
A four stroke piston engine of currant design is about 33% thermally efficient.While a two stroke is slightly less at about 30%.While a jet engine directly utilising heat for purposes of thrust is extreemly thermally efficient.

Due to parasitic power loss a four stroke piston engine achieves maybe 25% mechanical efficiency.(educated guess)
A two stroke engine would be better,at maybe something like 38% mechanical efficiency.(educated guess)
A rotory engine would be best.I believe they are near 40% in mechanical efficiency.
A jet engine is extreemly low in mechanical efficiency,unless it is of the turbine design which directly takes mechanical advantage of the thrust being generated.

A two stroke engine is only about 27% efficient in combusting gasoline.And around 29% efficient in combusting diesel fuel.
A four stroke piston engine is about 30% efficient in combusting gasoline.And around 37% efficient in combusting diesel fuel?
A rotory engine is about 29% efficient in combusting gasoline.(due to compression losses on the ends of the rotor.)
While a jet engine is somewhere in the 90% range for efficiently combusting fuel.

This is where I'm starting this discussion.If any of my information is flawed,please tell me so.

Now I come to the point where most people were misunderstanding me before.The primary reason for automobiles having catalytic converters and air injection systems into the exhaust is to burn the roughly 70% unburned hydrocarbons leaving the cylinders.Once past the converter,the fuel is over 90% combusted.But in the exhaust system,you get 0% energy gains.Some claim you can't burn vaporised fuel.That it can't be ignited with a spark plug.But the fact is you can run an engine on propane and methane both.They are both vapor fuels.A jet engine uses high speed air flow to mostly vaporize it's fuel.Which is high grade kerosene.I realise that an internal combustion engine can't run on completly vaporized gasoline because of tar and varnish residues left behind after complete vaporization.

In past times.It was believed you couldn't run a fuel air mixture in a carburated engine any leaner then 13.5:1.Any less would cause damage,and it did.Then electronic fuel injection came along.Instead of partially misted fuel from the carburator,we now had well misted high pressure fuel leaving the throttle body injectors or manifold injectors at a ratio around 14:1.Leaner mixture,same power,same combustion tempratures,but better combustion efficiency.Now we have direct injection of the misted fuel directly entering the combustion chamber.Having no chance to seperate in the intake stream anymore,we now have fuel air mixtures in the range of 14.5:1 and 14.7:1.
The combustion efficiency has been improved again with very slight thermal increases and slightly more power.

As fuel delivery systems have improved the quality of the fuel entering the combustion chamber,combustion efficiency has been improved.I know there are a great many factors involved in the overall efficiency of an engine.But I am trying at the moment to point out the combustability of gasoline fuel within a given engine design.To increase gasoline fuel percentage utilized,is in this case nothing more then the percentage of fuel burned in the combustion chamber.To know the percentage of fuel combusted in the cylinders,you would need an exhaust gas analyzer.And the EGR valve would need to be disabled,as well as the air injection,and the catalytic converter removed.

The only thing that would be proven analizing a car with all emission controls intact,would be the combined combustion efficiency of the engine and emission controls togather.

First of all, I dont know what the question is. But I dont belive combustion efficiencies are as low as 30% in a 4 stroke engine. Nor do I belive 70% of the fuel is unburnt. A rule of thumb I used to use was the 1/3rd rule. Which states that 1/3rd of the energy goes out the crank shaft, 1/3rd goes out the exhaust and the last 1/3rd gets lost in mechanical friction and thermal efficiency. This is probably way off today based on current technology.

In the late 50s the Wright aircraft engine company succesfully recouped a good portion of the exhaust energy by having the exhaust spin a turbine geared to the crank shaft (Turbo compounding). This recouperated an additional 700 HP per engine. These engines were also Turbo supercharged. These engines were succesfully used on the Lockheed Super Constallations. One of these engines is on display at the Smithsonian. In efficiecy these engines still slightly outperform todays turboshaft aircraft engines. In reliability, complexity and weight they loose badly.

I recomend books by Taylor, Heywood and one of the Bosch publications where these efficiency issues are discussed in depth.

Two good papers on friction from the SAE are:

"A New Approach to Evaluate Instantaneous Friction and its Components in Internal Combustion Engines" by Sohair F. Rezeka and Naeim A Henein, Wayne State University, Detroit, MI. 1984

"Effect of Design Variables on Friction and Economy" by I. N. Bishop, Ford Motor Company. Paper 812A, January 1964.

Also I agree, using an exhaust gas analyzer after the cat is of limited use. That is why some cars have a service port befor the cat. Even better is a port for each cylinder.

John Roncallo

Chris W. · #48 08-29-2005, 01:14 PM

I'm in the large diesel engine business - I have to guarantee efficiency values every day. Today's large diesels, both two stroke and four stroke, (I mean LARGE, like 10,000 - 100,000 HP) achieve an efficiency measured at the crankshaft of 50%. That is to say that for every 100 units of fuel energy burned in the engine, 50% goes to mechanical power. The other 50% is mostly rejected by heat. Heat into the block cooling circuit, the lube oil, and the charge air cooler (intercooler). Some heat is just radiated also.

The large diesel is the most efficient prime mover available commercially today. A smaller diesel might achieve a shaft efficiency on the order of 40 - 45%.

A gasoline engine such as those commonly used for automotive use is in the region of 30 - 35%, typically.

A gas turbine (i.e. "jet engine") is generally less efficient. In simple cycle use, which means no heat recovery from the exhaust, the most efficient gas turbine today is approximately 40 - 41% while most average considerably less. Some very small turbines, like helicopter engines, are down in the 28 - 30% range or worse. But large gas turbines used for power generation typically are placed in "combined cycle" wherein the hot exhaust goes through a boiler to make steam, which is then used to drive a steam turbine and make more electricity. Doing this, efficiency today can reach approximately 60%. This is only practical for stationary use.

Hope this info helps.

Rgds,
Chris W.
I do it for a living.

Ralph69220d · #49 11-09-2005, 01:16 AM

Quote:

Originally Posted by rallen

catalytic converters get vey hot, 400 C I was told. Given, they are not water cooled, only air cooled. But. If I took out the cat and replaced with steel tubing, would it also reach 400 C? If not then it would mean the cat converter is actually "burning" something present in the exhaust fumes.

From what little I can remember it uses some metals/alloys/ceramics to react chemically with the exhaust fumes and trap some CO or NO2 gases??

Yes, the catalytic converter is coated with a very thin layer of platinum. The cat converter "catalyzes a reaction), that is, it does something that would happen anyway without the cat, but at much greater efficiency. So, replacing the cat with steel tubing would not work. The cat converter get's so hot because the breakdown of these hydrocarbons into less toxic hydrocarbons is what is termed as and "exothermic" reaction, which means the energy from the chemical bonds, when they break, that energy is transformed into heat.

Ralph69220d · #50 11-09-2005, 01:25 AM

The large turbofan jet engine's used on commercial aircraft are very efficient in producing total work per energy consumed (fuel). But, somewhere around 70% of the power produced (work) by these turbofan jets is used to turn their own compressor's. Yet, they effectively gather and compress enough thin high altitude air to run well and the drag coefficients on the body of the plane start looking very good at 37,000 feet altitude.

Strife · #51 11-09-2005, 01:51 AM

I've always been interested in the valve train. I've read that a cam/valve train is really only optimal at one RPM and one load; if someone can design a reliable and cheap electric means of valve control, a lot could be done with the fast and cheap microcontrollers available today. So far, all we really have is shutdown control and that took 20 years (remember the 8-6-4 engine).

Given today's traffic conditions, if we could have an engine that could idle at a quart an hour, I think that effective gas mileage would go up a lot!

Interesting article:

http://www.greencar.com/index.cfm?content=features46

t walgamuth · #52 11-09-2005, 06:39 AM

of modern gas engines use variable timing on the valves.

formula one engines use pneumatic operated valves i think. and i think they may be electrically activated instead of with a cam, and if that is true then they can be individually controlled by a computer. and i hear that they are turning something like 20,000 + rpm.

internal combustion engines have come a long way since the 1930s when the highest tech was a supercharger and desdomonic valves on the mercedes grand prix engines.

fun discussion.

tom w

Ralph69220d · #53 11-09-2005, 09:16 AM

Quote:

Originally Posted by t walgamuth

of modern gas engines use variable timing on the valves.

formula one engines use pneumatic operated valves i think. and i think they may be electrically activated instead of with a cam, and if that is true then they can be individually controlled by a computer. and i hear that they are turning something like 20,000 + rpm.

internal combustion engines have come a long way since the 1930s when the highest tech was a supercharger and desdomonic valves on the mercedes grand prix engines.

fun discussion.

tom w

Yes, this is a fun discussion and I've learned a lot, too. I don't follow road racing, but I briefly watched one on tv a few year's ago. I knew the engine's were screaming compared to when I was a kid, but I had no idea they were up in the 20k range. Do you have an idea of what the stroke is on those engine's? I apologize for being too lazy to look it up, but could you explain the meaning of desdomonic as it applies to valves?

raymr · #54 11-09-2005, 10:28 AM

I can tell you that in reality, a rotary engine is horribly inefficient compared to a standard 4-cycle engine. True there is no valve train and only "3 moving parts", but they require huge cooling systems, oil coolers, and special exhaust parts to withstand the high heat loss. The reason for this is the fact that the combustion chamber actually travels around part of the housing with the rotor face, creating a large area of heat dissipation. The constantly changing shape of the chamber requires a richer mixture and 2 spark plugs for clean ignition, and the charge continues to burn as it leaves the exhaust port (ever see an RX7 shooting flames?). A piston engine in contrast maintains its heat in a relatively concentrated area. I do love the rotary sound at 8000 rpms though!

joselu43 · #55 11-09-2005, 03:14 PM

I suppose you mean "desmodromic". Applied to valves it means that there is a controlled mechanism that opens AND closes the valve as opposed to the valve being closed by a spring (no control).
As far as the stroke for F1 engines, I do not have any numbers, but it should be easy to get an estimate. These engines are "supersquare" mening that bore is larger than the stroke. They are 3 liters or 0.3 per cylinder. To start out you can assume bore and stroke are equal and go on from there. (pi X r squared X 2x r = 0.3, r = 3.62 cm wich gives a diameter around 3 inches. I did this quickly so it may be wrong. Check it out.)

JL

t walgamuth · #56 11-09-2005, 07:09 PM

oversquare. if the bore is 3 the stroke may be only 1.5. this allows the high rpm.

on the desmo valves, a little more detail: the cam has an extra lobe next to the one that opens the valve. there is a lever that attaches to the top of the valve somehow and the second lobe closes the valve positively. this stops valve float.

this was used as late as the fifites on the mb f1 engines. also ducati motor cycles have them to this day, i think. it is still a good system.

tom w

raymr · #57 11-10-2005, 12:17 AM

Quote:

Originally Posted by Strife

I've always been interested in the valve train. I've read that a cam/valve train is really only optimal at one RPM and one load; if someone can design a reliable and cheap electric means of valve control, a lot could be done with the fast and cheap microcontrollers available today. So far, all we really have is shutdown control and that took 20 years (remember the 8-6-4 engine).

Given today's traffic conditions, if we could have an engine that could idle at a quart an hour, I think that effective gas mileage would go up a lot!

Interesting article:

http://www.greencar.com/index.cfm?content=features46

I have thought about that too. It would open up all sorts of possibilities, like switching between 2cycle and 4cycle operation, reverse directions, shut down cylinders. They would have to be some powerful solenoids though. Much more than current 12volt electrical systems could handle. Maybe a new kind of springless valve design would be needed.

Jim H · #58 11-10-2005, 06:47 AM

Quote:

Originally Posted by Strife

...Given today's traffic conditions, if we could have an engine that could idle at a quart an hour, I think that effective gas mileage would go up a lot!

That is the whole idea behind a hybrid. Don't let the engine 'idle' but use a battery and electric motor instead. When the engine is running, let it operate at its most efficient for battery charging, then shut it off.

t walgamuth · #59 11-10-2005, 08:30 AM

i doubt that a diesel uses much more than that.

tom w

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