Since I consistantly see references to questions regarding mileage and power, I felt it prudent to open a thread that covers the subject in more detail. I will say that there are engineers and authors that have much more specific knowledge of certain aspects, but I would like to cover the fundamentals.

First and foremost is power. Power is a function of work over time. Keep this in mind when you watch dragsters or even better, when you modify your vehicle and compare your time slips to earlier ones, you see a quicker time. Your average power has increased, because your time is less.

Secondly is efficiency. An engine converts HEAT ENERGY into mechanical energy. It is interesting to note that gasoline engines are typically less than 30% efficient, and all that heat energy is blown out the tailpipe or is absorbed into the chassis and released into the atmosphere later through convective loss. There are many ways to increase the efficiency of your engine. More popular methods involve better lubrication, less restrictive air channels on the input and output side, and lighter components that move or rotate. Other means are controlling of the combustion of the atomized gasoline by quench, compression, pressure of the incoming fuel/air mixture, and spark timing to name a few. Altering any of the areas will result in an (assumably) increase in thermal efficiency. As the engines efficiency rises, the less power that needs to be converted to maintain the same power output occurs. Primarily, efficiency is increased the most through higher compression at the cost of fuels with a higher combustion temp (ie a higher RON or octane rating). Aluminum heads require more compression to compensate for the loss of heat around the cyclinder (this is covered in detail in professional writings and will not be covered here).

Finally, mileage is the distance covered with a finite amount of fuel. Since most vehicles use or used a mechanical odometer, one can test the mileage of a vehicle by topping off the fuel and using the mileage covered before topping off again to see what their "average" mileage is. The more miles covered combined with accurate fuel fill logs will constitute a more accurate "average" mileage.

Most commonly, I read that someone has put larger tires on their vehicle, and they freak out due to a change in their mileage. Well, since the circumfrence of their tire is larger (less revolutions per mile), and the weight of the tire is presumably more. The odometer cannot, without changing, account for the mass, nor the lessened revolutions per mile. To add to this, most people get comfortable driving at an assumed speed. For argument sake, we will use 60 mph as an example. If your tire diameter has changed and you continue to go 60 mph, a few things have changed. Most noteably, the air mass that you are pushing through is greater, the engine is at a different rpm (while probably lower, it may not be a more efficient rpm), and the mass that needs to move is presumeably greater. Other factors are the shape of your airstream (the change in the suspension may add to your slip), the amount of different driving conditions (ie more offroading), and the mechanical disadvantage of turning a larger diameter wheel.

I am a firm believer in less mass equates to more power (to the average of 50 lbs= 3 hp (or torque depending). Efficiency is very difficult to change in an engine due to the limitations of the metalurgy, mass of components used, and the engineering cost considerations of the manufacturer. Other factors include pressure or the fuel system (more pressure= better atomization), clocking and output of the spark timing, sealing of the components (rings, filters, quality and quantity of bends of components), and the quality and type of fuel used.

All in all your efficiency cannot be raised to a meaningful gain without excessive costs of reengineering components. Keep in mind that the fuel that you save must cost less than the components that you are changing to show any real advantage over time. The hybrid car is an excellent example as the offset in initial cost requires a few more years of use than the average consumers loyalty to car ownership.

Feel free to rant, argue, dismiss, discuss, or object to the above. If you do not agree, please support your objection. Also, if you feel you need to add to this, feel free.

 

Damn that is a good write up.

I agree and 30% efficiency of heat to mechanical energy. So what would you say a turboed engine efficiency is, which takes a little of the wasted exhaust to boost the air intake? Or better yet a Twin Turbo. 40-50%

How about an electric car?

 

best right up i have ever heard on the subject. great job siggy!

 

Turbo efficiency is measured after installation due to the multiple parameters that are affected. Also, turbo compressors work similarly to camshaft theory in that they are most efficient at a certain speed due to gas pressure laws. From my understanding though, an excellent increase in efficiency is around 10%, and 20% recycled energy is awesomeness.

Twin turbo theory versus single turbo theory differs a little bit, and they do not compound, unless they are parallel compressors. Sequential compressors have the benefit to cover a larger MAF range.

Electric cars are measure by the efficiency of the motor, and the distance of the associated wiring due to ohm loss. I studied motor theory for a few years, and the most I got from it was the more windings the better, and the more magnets the better. I would recommend looking at the exact motor in the vehicle and compare that to industrial data sheets.

Utimately, you can figure the Joules per gram of gasoline (based on what you use) and compare that to your torque ratings to see how efficient you engine/ driveline is. Bottom line is that is serves no purpose to change to a higher octane, if your timing/compression isn't adjusted to compensate for it. Positive displacement engine concepts recommend that your base engine be free from any inefficiency, because a net loss on a N/A engine is compounded in a positive displacement engine. All engines (and motors) start at 100% efficiency and through the designing, compensation, engineering, and cost associations we end up around 25-35% efficiency. I would recommend reading up on steam engine history to see why we have gasoline engines, and why we will ultimately move toward diesel engines (or diesel/electric). The steam engine engine turbine is the most efficient current technology on the planet as far as work (ie power plants), but not very portable.

EDIT: I forgot to mention that for every compression point, your engine's torque will rise approximately 4% (compounded). The use of aluminum heads neccessitate a higher compression due to the loss of latent heat, which is better for positive displacement engines if you maintain your compression. Mopar heads are oddities compared to GM and Ford heads, due to the camshaft location. They are not very flow worthy OOTB, and without moving the valve guides, meaningful gains are difficult. They were and are designed for torque in the lower RPM's so they have the street warrior covered. I hear alot of people mention portwork, and polishing work, but not much about chamber work (it can be costly), where the peanut shaped chambers can be unshrouded for the valves, and the best benefits are seen. I figure that a 10-15% increase in flow below .400 can be achieved here , but again, many are after peak power as opposed to average increases. Working the chambers will also increase the cc's to about 6-12 cc larger than stock, but again, the chambers can be balanced to work together and reduce stressing on the other engine parts.

 

good write up siggie. i would like to support what you said about larger tires lowering rpm to what may be a less efficient operating speed for the engine. i've got proof of that on my dakota. with 3.21 rear end and 27.5 inch tires i get 18 mpg. no easy driving either. that the correct tire height for it. now with 31's i got 15. yes that is an adjusted figure. i had to figure the percentage my speedo was off and then add that into the equation. at 65 i'm spinning around 1500 rpm. not efficient at all. around 1800 to 2200 would be a decent speed for it to run at. i'ts not always the engine speed that matters but it's the air intake. while it's spinning slower it's got to work alott harder which means more air and fuel (verified watching a vac gauge and the injector pulse width if you want to try it out). when it's turning faster it's not working as hard so it doesn't need as much fuel.
as for the heat being how it works that's 100% correct to. someone at work read this and had a hard time understanding that. the chemical energy is released by the boom when the plug fires. the expanding gasses push the piston down. now he had time understanding that meant the heat. hot air takes more space and i wanted to point that out for anyone else that may have to think for a few.

all of these things come out to a few proven points.
these engines (3.9 and 5.2 and 5.9 mags and la's) are meant for torque. they do well at that. that why some ppl complain that at 5 grand it's not very strong. well the engine doesn't breath well at that speed. now slap 3inch pipes and a m1 intake on it and it does better. now when you go to tow with it your mileage will be less because at those lower speeds the air flow in the manifolds is slower which changes how well it can breath. that's why myne still has the 2.5 inch pipe on it. i tow with it and i could tell a diff from the 3 inch pipes it had when i got it to the 2.5 inch pipes it has now. it's the air speed. now if i race one of you guys with an identical truck but with 3 inch pipes ill most likely loose. but if i tow up a mountain myne will do it easier because the exhaust gasses are travelling faster wich allows the chambers to more completely empty of exdhaust gasses and to fill more with fresh air.

 

Yep, to add to this, the current supporting theories are that the reversion waves support exhaust gas pulses (which you want to help the escaping gases in relation to crank position). The exhaust is of a standard quantity, so you can change the length of your expansion joint to compensate the reversion wave (ie tuned header length). Again, the distance will change the rpm that benefits most from the venturi effect of the expansion points. A muffler really messes up the equation if it is not a straight through type.
Also, to add to the "spark go boom", colder air and colder fuel will cause a vacuum effect (will aid in "pulling" the piston) within the cylinder chamber. This benefit is difficult to produce meaningful gains (especially in the upper rpm range due to the speed of the effect), unless a stronger ignition is used to compensate for the reduced energy stored in the air and fuel. At the drag strip is the most useful gains, and your timing can be advanced if you control the temps of the incoming fuel and air. I would not recommend adding a fuel cooler, or a CAI for a stock vehicle as the gains in efficiency do not support the cost of the parts for a typical period of ownership.

As for torque curves versus horsepower curves, the stroke length affects the torque curve the most, whereas the bore diameter affects horsepower the most. This is not taking into considerations of valve timing and duration. Thanks for mentioning it. Another thing I find on the forum is the "overporting" of the intakes. The intakes are not very well designed and I would strongly encourage people to purchase a dual plane intake that does not have a soot trap at the bottom. This of course is only for the lower RPM crowd where their average rpm is in the 2-3k range. Only porting it to remove casting slag and balancing the ports. Head porting for most (street duty) should also be limited to removing casting flaws, tapering the valve shroud (intake and exhaust), cc the chambers with valve deshrouding (professionally only or with a machine- not by hand), and polishing the exhaust to about 300 grit. Any excessive work on the intake side will only affect the lower rpm velocity of the incoming charge, and possibly make the engine weaker in the low RPM range.