I have some one I work with who insists gasoline engines only burn 25 % to 35 % of their fuel and the rest is expelled with the exhaust and burned in the catalytic converter . The sad thing is this guy is a mechanic .
I believe they burn 97 % or more of their fuel .
Any one have any help full links ?

 

Your mechanic is confusing percent of the fuel burned with the thermal efficiency of an internal combustion engine. Wikipedia has a pretty good discussion on this.

http://en.wikipedia.org/wiki/Thermal_efficiency

Your mechanic probably has not taken thermodynamics classes, so I can understand his confusion. I have tried taking a thermo class but it took me more than I took it.

You are correct that almost all of the fuel combines with oxygen and burns in a modern factory engine. I guess if you had a carbed gas race engine running pig rich and blowing black smoke you could say that some percent of the fuel wasn't being burned. If 65-75% was just passing through the engine it probably wouldn't run very long.

No offense, but I would chose another mechanic to work on my car.


Joe Urban

 

I wouldn't sweat finding a new mechanic. Just because he didn't take Physical Chemistry (a 300-level chem course... probably more to the point than just a Thermodynamics / Physics 200, course... if you didn't like Thermo, you'll REALLY HATE P-chem ), it doesn't mean he can't follow a diagnostic manual/equipment.

The high-pressure/high-temp chemistry involved in combustion is even beyond P-Chem... more likely a graduate-level investigation. You don't need that kind of theoretical research understanding to tune or assemble an engine based on an existing design. Some of the very best engine builders have no formal college classes but EXTENSIVE tuning/mod'ing experience who have seen what does and doesn't work on certain combinations.

I don't know jack about OilPan4's mechanic, but I don't think his understanding of chemical potential conversion to kinetic energy efficiency would hurt or even help his tuning abilities.

 

Look at the graft in on this site, it shows a BBC with 100% mass fraction burned
by 85 degrees after TDC. There is no set number, every engine is different.
Even the same engine will be different under different operating conditions.

I believe your mechanic confused % of fuel burned with engine efficiency.

tfxengine.com

Production engines need fuel in the exahust to fire the cat. The Xb line on this
graft is a representation of mass fraction burned in a production engine ~ 95%.

 

Just like an incandescent light bulb, it burns 100% of the fuel but only uses a percentage of that energy to produce work. The rest is lost as heat or lost due to friction. A frictionless engine that produces no heat would be 100% efficient.

 

No offense was intended to anyone. Without personally knowing the mechanic in question, I would not presume to advise others.

FWIW, Mr. Steve, I did not advise oil pan 4 to change mechanics, just that I, personally, would change. I, personally, have a problem hiring people to perform work on my belongings or my body when they insist that what they know is true when it is not. That tells me that they may have mistaken ideas about more practical things and also may have a closed mind. That is my experience in the years I have been dealing with people. Disclaimer: your experience may differ.

Joe Urban

 

I'd be a bit suspicious of a mechanic that made such a statement. Doesn't mean he can't mechanically assemble parts and build killer engines, but I'd hesitate to trust him with tuning. JMHO.