Automobile HP and Gas mileage question

• Automotive
• gtacs

gtacs

Hi all,
I have a few questions concerning cars and HP, I will post the first question now and I think the answer will determine if I need further help. I will prefeace my question with the disclaimer that I have very limited automotive knowledge.

If 2 cars that are operating at the same HP, are they consuming that same amount of fuel, assuming they have the same engine efficiency? Would it matter if one vehicle had a turbo/supercharger?

same HP, are they consuming that same amount of fuel,
Same mass/weight?

Same mass/weight?

I hadn't thought of that. I was focused on the engine itself, not the vehicle. But it is a good question.

Lets say the vehicles are not moving and putting out 500 HP.

So, let me give you a little history on why I'm asking. I watched a youtube video on a guy who took a honda civic and did engine modifications (turbo, supercharger etc..) to get 500hp. I have also seen other muscle cars put the same 500hp out. Typically, a little 4 cylinder engine would use allot less fuel than a big v-8, the 4 cylinder would typically have less hp (according to my understanding). So i was just curious if 2 cars putting out the same HP consume the same amount of fuel. I see now that vehicle weight/gearing etc could effect gas consumption, so let's say they are not moving?

Going back a bit in history. A 2007 Porsche 997 turbo makes 473 hp from a twin turbo 3.6 liter engine. It weighs about 3400 lbs and gets around 25 mpg when cruising at highway speeds. A 2006 Corvette Z06 makes 505 hp from a naturally aspirated 7.0 liter engine, which is lighter than the Porsche's 3.6 liter engine and turbos. It weighs about 3150 lbs and gets around 25 mph when cruising at highway speeds. What I don't know is what the fuel consumption rate is with both engines running near max power output at say 400 to 450 hp.

out 500 HP.
Equals product of thermodynamic efficiency, twenty to thirty percent, and the chemical energy given/fed to the engine.

We need to be more specific on the fuel consumption question.

You can make 500 HP with a huge cubic inch normally aspirated V8 engine or a small cubic inch engine that is supercharged or turbocharge. Both will have maximum horsepower of 500 but at differing engine crankshaft speed. The carbureted 500 cube engine would run a maximum of 6000 rpm while the little turbo formula car engine will go 12,000 rpm and really rattle your teeth. Both will have different rpm when idle or under zero load. Both will have different fuel consumption through the entire rpm range.

This is a microcosm of the evolution of the passenger car industry since the first oil embargo in the 1970s. Detroit killed off the huge (400 cubic inch displacement and up) engines due to cost of manufacture, weight savings and fuel mileage. Gas was no longer 25 cents a gallon.

The Detroit big blocks weighted in at 500 + pounds. Took massive heavy drive shaft, transmissions rear differentials axels and the like. The first thing to go when looking for fuel mileage is weight. Eventually the V8 was killed off except for performance cars like Mustang, Camaro, police cruisers and pick-up trucks. The whole car size was down sized to more compact frontal areas to chat the wind resistance. Aero dynamic became critical for fuel mileage. More effective engine ignition and fuel management came about to increase fuel mileage and conform to regulations on pollution. Now the typical grocery getter has the same emissions idling as a human jogging.

I think what your real question is with 2 engines of 500 hp which has better fuel mileage.

The lightest weight engine package in the lightest vehicle with the least aero drag (CD) and best transmission gearing will have the best fuel mileage. It will not necessarily be the fastest vehicle however.

Lot of things to consider and welcome to the forum.

russ_watters
A key measure of engine efficiency is BSFC (Brake Specific Fuel Consumption). The units are either pounds of fuel per horsepower-hour or grams of fuel per kilowatt-hour. Here is a thread with a collection of BSFC maps: https://ecomodder.com/forum/showthread.php/bsfc-chart-thread-post-em-if-you-got-1466.html.

And here is a collection of MPG vs speed charts: https://ecomodder.com/forum/showthread.php/speed-vs-mpg-charts-post-em-if-you-15182.html. Plus the 505 hp Corvette MPG chart that every person clearly needs: https://ecomodder.com/forum/showthread.php/tested-speed-vs-mpg-2008-corvette-z06-505-a-9841.html.

jim hardy
If 2 cars that are operating at the same HP, are they consuming that same amount of fuel, assuming they have the same engine efficiency?

Short answer is yes. Provided they both use the same definition of efficiency.

efficiency (in %) = 100 * power out/power in

If the efficiency is the same and the power output is the same then the power input must be the same. However that doesn't mean the fuel consumption is the same as not all fuel is the same.

The amount of energy in a defined amount of gasoline varies for a number of reasons. To compare like with like car makes have to agree how much energy is in a gallon of gasoline. Fortunately government's issue standards.

Here in the UK home heating furnaces can have an efficiency of over 100% (eg 104%) because the definition of how much energy is in a fixed amount of natural gas assumes some energy lost out the exhaust is never available. Now we have condensing boilers some of the heat previously lost can be recovered leading to results that appear to defy the laws of physics.

We need to be more specific on the fuel consumption question.

You can make 500 HP with a huge cubic inch normally aspirated V8 engine or a small cubic inch engine that is supercharged or turbocharge. Both will have maximum horsepower of 500 but at differing engine crankshaft speed. The carbureted 500 cube engine would run a maximum of 6000 rpm while the little turbo formula car engine will go 12,000 rpm and really rattle your teeth. Both will have different rpm when idle or under zero load. Both will have different fuel consumption through the entire rpm range.

This is a microcosm of the evolution of the passenger car industry since the first oil embargo in the 1970s. Detroit killed off the huge (400 cubic inch displacement and up) engines due to cost of manufacture, weight savings and fuel mileage. Gas was no longer 25 cents a gallon.

The Detroit big blocks weighted in at 500 + pounds. Took massive heavy drive shaft, transmissions rear differentials axels and the like. The first thing to go when looking for fuel mileage is weight. Eventually the V8 was killed off except for performance cars like Mustang, Camaro, police cruisers and pick-up trucks. The whole car size was down sized to more compact frontal areas to chat the wind resistance. Aero dynamic became critical for fuel mileage. More effective engine ignition and fuel management came about to increase fuel mileage and conform to regulations on pollution. Now the typical grocery getter has the same emissions idling as a human jogging.

I think what your real question is with 2 engines of 500 hp which has better fuel mileage.

The lightest weight engine package in the lightest vehicle with the least aero drag (CD) and best transmission gearing will have the best fuel mileage. It will not necessarily be the fastest vehicle however.

Lot of things to consider and welcome to the forum.

Thanks for the reply Ranger Mike. At the 500HP would both cars consume the same amount of gas if both cars were sitting still and the engine efficiency was the same on both engines? It seems like they would. I mean if one engine is at 12000 rpms (even though it uses less gas it is going twice as fast) and the other at 6000...

At the 500HP would both cars consume the same amount of gas if both cars were sitting still and the engine efficiency was the same on both engines?

I can not answer this question because it is too subjective.

Lets narrow this down a little. There are several definitions of engine efficiency ..here are the most common-

Volumetric efficiency - Is the ratio of volume of air sucked by the engine to volume swept by the piston . Piston moves towards bottom dead center in the expansion stroke but the volume it sucks from outside is not the same as the swept volume because the fuel / air ration changes this by expanding. The volumetric efficiency depicts this.

Mechanical efficiency- It shows how efficiently the engine converts one form of energy to another form by taking losses like friction, leakages into account. Like conversion of indicated power to brake power with minimum losses shows the mechanical efficiency of engine

Thermal efficiency -. It is the amount of work that can be obtained by using unit amount of heat energy .

This is the most popular - There is a lot of concern nowadays about the thermal efficiency of the internal combustion engine (ICE), and a lot of research is being done to improve it. But what exactly is the efficiency of the internal combustion engine and how do we measure it? The efficiency of any engine is simply calculated from the energy of the fuel supplied per unit time to do work and the output at the shaft of the engine after subtracting all losses. The input power of the fuel can be obtained from the mass of the fuel and its calorific value. The shaft output can be measured from a brake dynamometer. Simply put efficiency is Output/Input. The average ICE has an efficiency between 20 to 30%, which is very low. If we see a heat balance sheet of the internal combustion engines for a spark ignition or gasoline engine we find that the brake load efficiency is between 21 to 28%, whereas loss to cooling water is between 12 to 27%, loss to exhaust is between 30 to 55 %, and loss due to incomplete combustion is

between 0 to 45%. Similarly when we analyze the heat balance sheet of a compression ignition or diesel engine we find that it has a brake load efficiency between 29 to 42 % and loss to cooling water is between 15 to 35 %, losses to exhaust is between 25 to 45 %, and losses due to incomplete combustion is 0 to 5 %. By analyzing the two heat balance sheets we find that in Gasoline engines loss due to incomplete combustion can be rather high.

Diesel engines up to 35% in best point, gasoline engine up to 30% in best point. In real life use, averaged about 25% for drivetrains with Diesel and about 20% for those with gasoline engines.

The engine efficiency depends on the engine rpm and load. Car engines rarely operate at their peak efficiency, so peak efficiency and rated power are of limited relevancy to the reality.

The above-mentioned efficiency are basically measure from tank to crankshaft (thermal efficiency). In reality, there are more lost along the way; in flywheel, transmission, axle etc. before power reaches the wheels. So effectively, the tank-to-wheel efficiency is in the order of about 10% or much less (quoted from Prof. John Heywood's article some years back).

Regarding the two engines at 500 hp scenario – one normally aspirated big cubic inch engine versus the little high revving 12 cylinder turbocharger mill.

turbochargers do offer efficiency gains. But that efficiency can quickly disappear if you step on the gas too quick! A turbocharged engine turns into a fuel-hog under hard acceleration, because the large volume of air being pumped into the cylinders must be matched by a larger volume of fuel.

This abnormally high fuel consumption of a turbocharged engine under high load is a matter of survival. To perform properly an engine must mix air and fuel in a precise ratio. The perfect air/fuel ratio is about 14.7 parts of air to one part of fuel. This is known as a "stoichiometric" ratio, which ensures a chemically complete combustion event. If you introduce more fuel than necessary, you create a "rich" mixture, and part of the fuel passes through the engine unburnt, wasting gas and creating extra pollution. A lean mixture, on the other hand, saves fuel, but makes the engine run hotter. A turbocharger compresses the incoming air, the fuel injection system automatically adds more fuel to keep the mixture at the correct ratio. And this is where the problems begin. As the pressure in the combustion chambers rises, you run the risk of pre-ignition (commonly known as "knock") – this is caused when raw ignites before the spark plug fires. DETONATION! Boom. Instant boat anchor. Knock is destructive to everything but the automobile dealership repair shop! The good news is that it is easily prevented by computerized engine control systems that monitor fuel flow and cylinder in real time. If your engine is on the verge of knocking, the computers have an instant fix: they shoot extra fuel into the engine. There goes the fuel mileage.

I can not imagine an engine making 500 hp at 6000 rpm will have the same thermal efficiency (fuel consumption) as one making 500 hp at 12,000 rpm.

https://www.quora.com/Why-is-the-ef...nes-poor-when-they-are-running-at-high-speeds

Asymptotic and gmax137
You can make 500 HP with a huge cubic inch normally aspirated V8 engine or a small cubic inch engine that is supercharged or turbocharge. Both will have maximum horsepower of 500 but at differing engine crankshaft speed. The carbureted 500 cube engine would run a maximum of 6000 rpm while the little turbo formula car engine will go 12,000 rpm and really rattle your teeth.
The 2006 (and later) C6 Z06 has a 427 (7 liter) cubic inch push rod engine that makes 505 hp and redlines at 7000 rpm. As I posted above, the Z06 engine is lighter than the 2007 Porsche 3.6 liter dual overhead cam engine and it's twin turbos, that makes 473 hp and redlines at 6750 rpm. Chevrolet was intending to use another high revving big displacement engine in the C7 Z06, but it couldn't meet emissions, so they switched to a 376 cubic inch (6.2 liter) supercharged engine that makes 650 hp and redlines at 6500 rpm (the redline apparently chosen due to emission limits not design issues.)

The current Formula 1 turbocharged 1.6 liter engines are allowed to run up to 15,000 rpm, but ended up reduced to running at around 12,000 rpm for better mileage when the fuel loads were reduced to 100 kg (it use to be higher for the prior 2.4 liter V8's with a 19,000 rpm rev limit due to rules). For some reading this, Formula 1 cars don't refuel during a race, they start off with all the fuel they will use in a race, and the pit stops are only done to replace tires (unless body parts are replaced due to damage).

https://en.wikipedia.org/wiki/Formula_One_engines

Back to the topic, with a turbo charger, some of the otherwise wasted heat energy is captured by the turbo, but in most cases where the turbo charger is being used to increase power, the turbo charger consumes power by restricting the exhaust of an engine. I don't know if there is an inherent difference in efficiency between the two engine types, most likely it is specific to the actual engines involved.

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A rough rule of thumb is that a gasoline engine set up for economy will burn 0.4 lbs of gasoline per horsepower-hour, and 0.5 lbs/hp-hr with a richer mixture for power. This assumes that the engine is operating somewhere near its best efficiency point. The efficiency decreases at low RPM, and less than about 50% torque. At idle, of course, the efficiency is zero.

500 hp from an automobile gasoline engine is about 40 gallons per hour, regardless of cubic inches or RPM. A car putting out 500 hp will very quickly be at a speed where gas mileage is irrelevant. My truck, for example, needs only 20 hp to the rear wheels to maintain 55 MPH on a summer day with a calm wind.

jim hardy and russ_watters
I can not imagine an engine making 500 hp at 6000 rpm will have the same thermal efficiency (fuel consumption) as one making 500 hp at 12,000 rpm.

I have not been clear in my concept of fuel consumption. I wholeheartedly agree that if two engines make 500 hp and have the same efficiency the fuel used will be the same ( at this 500 hp point). It takes a certain amount and quality of fuel (gasoline) to attain the horsepower and this is pretty much a settled issue.

Being an old racer, i am thinking of the fuel consumption over the entire race and was not clear in my statement. Sometimes i get to focused in on my ideas and do not have the luxury of my fellow forum contributors and their perspectives of stepping back and taking a look from an educators point of view.
nuff said?
happy new year
rm

jim hardy
A rough rule of thumb is that a gasoline engine set up for economy will burn 0.4 lbs of gasoline per horsepower-hour, and 0.5 lbs/hp-hr with a richer mixture for power. This assumes that the engine is operating somewhere near its best efficiency point.
Another easy rule of thumb is that 2 gal/hr is 30 mpg at 60 mph.

If 2 cars that are operating at the same HP, are they consuming that same amount of fuel, assuming they have the same engine efficiency?

Ranger Mike gave an excellent answer here:
Simply put efficiency is Output/Input.
Output is 500hp
Input is fuel flow rate
Efficiency is their ratio

Applying first year algebra
##Efficiency = \frac{Output}{Input}##
multiply both sides by Input, divide both sides by Efficiency
##Input = \frac{Output}{Efficiency}##
Efficiency you didn't define but you said it's same for both engines
so the two ratios ##\frac{500}{Efficiency}## are of course the same because they both have same numerator and denominator
meaning the inputs, fuel flow rate(aka fuel consumption), are also equal

So I'm not at all sure what was really your question...

Words have various meanings in various circles.
Ranger Mike gave you a definition of 'efficiency' as it's understood in scientific circles
What is your concept of efficiency ?

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If 2 cars that are operating at the same HP, are they consuming that same amount of fuel, assuming they have the same engine efficiency?
The quantity of energy that 1 kg of fuel burned will release can be found with the Low Heating Value (LHV) of the fuel. For typical gasoline & diesel fuels, the energy released during the combustion of 1 kg of fuel is about 40-45 MJ. That energy is in the form of heat. An engine transform heat energy into work which have the same unit (Joule). If the work output of the engine is 42.5 MJ for every 1 kg of fuel burned, then the engine has an efficiency of 100% (assuming LHV = 42.5 MJ/kg).

Multiplying the LHV by the mass flow rate (in kg per second), you get the heat power due to the combustion. So gasoline burned at a rate of 1 kg/s will produce 42.5 MW of heat energy (= 1 kg/s X 42.5 MJ/kg). That corresponds to 56 970 hp of mechanical work for every 1 kg/s of fuel burned.

So, for example, if you want 500 hp from an engine that converts heat into work at an efficiency of 30%, what mass fuel rate do you need?
$$\frac{\left(\frac{500\ hp}{56\ 970\ \frac{hp}{kg/s}}\right)}{0.30} = 0.029\ kg/s$$
In an earlier post, someone said something about engines requiring 0.4-0.5 lb/(hp.h) of fuel. Where does it come from? Well, we said gasoline produces 56 970 hp of mechanical work for every 1 kg/s of fuel burned, or 7937 lb/h. Thus, if we assume an engine has a 30% efficiency, it should require the following mass flow rate per horsepower:
$$\frac{7937\ lb/h}{0.30 \times 56\ 970\ hp} = 0.46 \frac{lb}{hp.h}$$
As you can see, assuming the same efficiency, a 500 hp engine consumes the same amount of fuel compared to any other 500 hp engine.
Would it matter if one vehicle had a turbo/supercharger?
The real question here is: Does a turbo/supercharged engine has the same efficiency as a naturally aspirated engine?

This question could be tricky to answer if the engines are very different in construction. So many other variables can influence the efficiency.

That being said, if we take a naturally aspirated engine that produces 500 hp at wide-open throttle and we add a turbo/supercharger to it (even assuming appropriate intake/exhaust tuning), we could restrict the throttle opening to still get 500 hp. If both engines have the same efficiency, they will consume the same amount of fuel. But in such a case, the naturally aspirated version will probably (surely, in the case of a supercharger) be slightly more efficient because the compression is completely done within the cylinder, creating less losses.

jim hardy
As already mentioned, if the output power is the same and the efficiency is the same then the fuel input must be the same, this is by definition.

Turbos and to some extent super chargers allow you to play with that efficiency, you can either get a lot more performance out of a given engine volumetric capacity or you can increase the efficiency of that engine, or a bit of both. Turbos basically use the waste heat from an engine and use that mechanical work to force air into the intake. This does two things, you get more air in the same volume and can therefore burn more fuel, and due to increase in compression ratio burn that extra fuel more efficiently (=more power from given engine capacity) or, you can just effectively increase the compression ratio of your engine, thereby increasing the thermal efficiency, ie an engine will give you more work for the same amount of input heat (fuel).

This is why turbos are prevalent everywhere from competition cars to stationary industrial diesels.

Both turbos and supercharges create a parasitic load on the engine, either via pumping loss (turbo) or direct mechanical loss (super charger), however the reason both are done is because the power and efficiency gains far out weight the extra loss.

In the end, to come back to your question, a turboed engine, unless you've done something bad, should be more efficient than a NA engine therefore will use less fuel for the same power.