A discussion on engine efficiencies

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.

[t walgamuth]

i missed the earlier discussion. is it worth looking up? what is the name of the thread?

tom w

[Jim H]

Originally Posted by Rick & Connie

...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...

I agree that the catalytic converter gives no power gains.

I must respectfully disagree with the 70% unburned figure out of the engine.

If the exhaust contained 70% unburned fuel, the catalytic converter would be burning MORE fuel than the engine. If this was true, the converter would thus reject more heat than the engine, and probably burn up the car in doing so.

I will try to find the unburned HC figures before and after the converter.

Originally Posted by Rick & Connie

...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.

If you are referring to my post, I said that a spark plug cannot ignite a 'lean' fuel/air mixture. I didn't say anything about 'vapor' one way or the other.

A gas turbine engine injects kerosene into a combustion chamber. The air in the chamber has to be slowed down enough that combustion can take place.

Originally Posted by Rick & Connie

...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.

I will agree that a carbureted engine cannot run with a fuel/air mixture that is as lean as a fuel-injected engine. Yes, direct injection gives even better control, and it can be even a bit 'leaner.'

[Deleted by editing, erroneous information]

Engines today can burn a bit 'richer' to produce power, since they leave the cleanup of emissions to the cat. Too much unburned fuel, such as a spark plug that does not fire, will allow too much fuel into the converter and it will overheat.

Best Regards,
Jim

[Moneypit SEL]

If you wish to be taken seriously, you MUST post verifiable information instead of your 'educated guesses' and beliefs. In other words, do your homework, look up the information, learn it, then form your opinions accordingly. It is far too tedious a task to enter into a discussion where 'guesses' are elevated to fact status. Especially when, in the face of irrefutable information to the contrary, you cling to your guesses and beliefs with the tenacity of a pitbull clinging to a porterhouse steak.

[mattplews]

Personally,

I do not like Catalatic Convertors, Here in the Uk if we want more power we remove the cat and have a straigh through exhaust fitted

Noisy but about 15-25bhp increase

Dont know if thats really realated now

[manny]

Originally Posted by mattplews

Personally,
Noisy but about 15-25bhp increase

If your old one is plugged up............maybe.
Never seen a converter yet that " costs " more than 5 hp.

Furthermore, of ALL PLACES, to remove a catalytic coverter in Great Britain, or anywhere else in Europe, is socially & environmentally irresponsible.

Originally Posted by Rick & Connie

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.

This is the piece of info I'm having a problem with. Is it your understanding that a 4 stroke gas engine only burns 30% of the available fuel in the combustion chambers, and that 70% is unburned and goes to the catalytic converter or the environment? If that's what you're saying, I would respectfully ask for the basis of that information. I believe you have some incorrect information. I could not find a reference for the normal percentage of Incomplete Combustion in a 4 stroke gas engine, but I have one reference that states it is "very small." My best guess is that the amount of unburned fuel leaving the cylinders is a couple of percent or less. I suspect the 30% value you are quoting is an approximate value for the overall engine efficiency, including combustion, mechanical, and thermodynamic efficiencies.

It may well be that I've misunderstood the 30% efficiency I've read in several places.Maybe the number was supposed to represent overall efficiency,and wasn't clearly stated.But this is what I wanted.Someone telling me where I'm wrong.Not just dismissing everything.If I've made a mistake,I want to know where it is.Thank you.I misunderstood overall efficiency figures to represent combustion efficiency figures it seems.But other then that,am I in the right ballpark in understanding the other basic comparisons with different types of engines?
Tom W.the thread that origionated this was "Regular instead of Super!???".
It started about fuel saving ripoffs.I described the most common one that keeps popping up useing magnets on the fuel line,and brought up two vapor carb designs I know of that seem to work fantasticly.I got ridiculed some and got very frustrated over the matter.I admitted that in my opinion,at least 80,000 miles worth of testing should be done before anyone could say any vapor design can be said to work exactly as claimed.Now I'm trying for a good discussion of the three main factors determining engine efficiency.So far so good.
I'm currantly suffering due to broken back and fractured neck pain,which makes concentration difficult at times.So I'm sorry to all here that I can't remember exact numbers alot of the time.Or where some of my info originated anymore.But I still remember all the basic theories.And I remember all the applications to my many years of experience as a mechanic.That's why I can't make exact quotes for you here in most cases.But I'm not ignorant either.And I'm more then willing to learn what I don't know,or be corrected where I may have confused some facts.

[t walgamuth]

1. a 14.5 to one ratio is leaner because the larger number is for air.
2. a 30% effecient engine only makes 30% worth of mechanical energy from 100% of the energy in the fuel. that does not mean that 70% of the fuel is unburned. i would guess that that number is way less than 5%.

sorry to hear about your broken back.

my $.02

tom w

[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??

[manny]

Originally Posted by rallen

But. If I took out the cat and replaced with steel tubing, would it also reach 400 C?

NO.
Because you would eliminate the " Catalytic Action " of the converter which generates these temperatures, & therefore the temperature of the replacement pipe would be much cooler.

[Ralph69220d]

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.

[Jim H]

Originally Posted by t walgamuth

1. a 14.5 to one ratio is leaner because the larger number is for air.

Tom W, thank you, I remembered and spoke incorrectly. I said fuel/air ratio but industry uses air/fuel number. My bad.

The following is a link to an interesting article about measurements taken at various operating stages of a catalytic converter. Shows various inputs and outputs, under start-up, normal operation with and without the air injector pump, and operation with 1-cylinder misfire. I was interested in the HC numbers.

http://www.asashop.org/autoinc/oct97/techtotech.htm

Best Regards,
Jim

[t walgamuth]

burns the fuel residue in the exhaust. it is a ceramic element that gets very hot and then burns things. some wood stoves also have a cat in their flue. it does the same thing and eleminates pollutants. in this case the heat given off is useful for heating the house.

tom w

Originally Posted by Rick & Connie

I misunderstood overall efficiency figures to represent combustion efficiency figures it seems. But other then that,am I in the right ballpark in understanding the other basic comparisons with different types of engines?

If we are talking about overall efficiency, I think the numbers you quoted (around 30%) look reasonable, except for the 90% jet engine. The 90% value may actually be a combustion efficiency number, it is much to high for the overall efficiency. There are lots of different types of "jet engines" and I'm not an expert on any of them. I don't have a source to verify the efficiencies of current engines, but I wouldn't be too surprised if the latest high efficiency engines (gas and diesel) were doing a little better. Maybe someone else knows.

Getting back to basics for a minute, someone pointed out that in an internal combustion engine about 1/3 of the energy is lost to the environment as heat (mostly though the cooling system), about 1/3 is lost out the exhaust (as hot gasses), and about 1/3 is turned into useful (shaft) power. These are round numbers, but close enough. Obviously, our goal is to minimize the lost energy.

If we look at the 1/3 that is lost as heat to the environment, we can theoretically get rid of the cooling system and operating the engine at higher temperatures, except it will melt . This is where the proposed ceramic (and ceramic coated engines) come in. The idea is to design an engine that will operate reliably at higher temperatures. I don't have any problem with this concept, but (as far as I know) no-one has developed a practical, cost effective design. I think a lot of the issues have to due with trying to manufacture complex ceramic parts (e.g., space shuttle tile problem). As materials improve and engines can be reliably operated at higher temperatures (for reasonable costs), this loss should inch down.

The 1/3 that is lost out the exhaust (as hot gasses) is pretty much a given. The second law of thermodynamics limits the maximum efficiency between a high temperature source (combustion temperatures) and a low temperature sink (ambient temperature). As we discussed earlier, this maximum efficiency is about 70% based on assumed temperatures of about 1300F and 70F. That means we are going to lose at least 30% no matter how well we design the engine (no free lunch there).

If we consider the 1/3 that is lost to cooling (material limits) and the 1/3 that is lost through the exhaust (thermodynamic limit), we don't have a lot left to work with. All modern engines are designed to provide maximum combustion efficiency (while still meeting emission and temperature limits). Current fuel injection control systems are very complex and are designed to continuously optimize the combustion process.

If we go back to the 50s and 60s there was clearly room for improvement in the carburetors. At that time, I can believe that someone could design a carburetor that would be a significant improvement over the stock designs. I also understand that a carburetor could be designed/adjusted to operate very lean, resulting in higher operating temperatures and an improvement in efficiency. However, if the engine was not designed for those operating conditions it would not be reliable. In addition, there are emissions consequences (i.e., high NOX) associated with high combustion temperatures that would have to be addressed.

With regard to the "vapor carburetor" design, I'm not sure I understand what it was trying to do. I suspect it was designed to operate the engine under very lean conditions, minimizing incomplete combustion and increasing operating temperatures. I expect this would result is an increase in efficiency (I don't know how much), but I also expect it would be very hard on the engine and would adversely affect some emissions. If that was the goal, the same thing could be accomplished today by reprogramming the fuel injection system on a modern engine.

If I understood your earlier posts correctly, you were attributing the increased efficiency of the "vapor carburetor" design to minimizing incomplete combustion. I doubt that was the major effect because stock engines should have very small loses due to that. If this design was causing the engine to run very lean (and hot) it could have increased efficiency in the short term. If that is the case, this design concept has been superseded by modern fuel injection (and engine management) systems that allow all these variables to be continuously monitored and controlled.

Personally, I think internal combustion engines have reached the point where improvements in efficiency will come very gradually, because all the easy stuff was done years ago. Engine management systems and materials will continue to improve, but I would be surprised to see a sudden leap in efficiency. I suspect we will see bigger improvements in the remainder of the vehicle (lighter weight, improved drive-trains, etc.), but that's a whole other subject.

What do you think?