What determines overall engine power?
Lomac
05-14-2004, 12:46 AM
My friend has told me that Inline 6 cyl. engines produce more power than a V6. Is this true? And what determines the power that an engine can and will produce? Why does a lower reving v8 produce so much more power than a V4 that runs at 6000 rpms? If the same amount of fuel is burned shouldn't the same amount of power be produced? Any insight you can give me in this subject would be appreciated.
59elcooldsuv
05-14-2004, 09:47 AM
This series of questions calls for an in-depth discussion of the basic differences between Torque and Horsepower. You're just going to have to forgive the fact that the answer is going to be long-winded.
Generally, the lower-revving engine has been designed to make a lot of torque at low RPM while the hi-revving engine has been designed to make HP at high RPM. It's not quite like comparing apples to oranges - it's more like comparing big, heavy Clydesdale draft horses (think; Budweiser beer wagon) to small, sleek, fast racing horses. You can have one or the other, but usually not both in the same animal. Of course, you can have an animal that is a reasonable compromise between the two, but it won't be as fast as the race horse and won't pull wagons like the draft horse.
Torque is the ability to twist a shaft, regardless of how long it takes to make a revolution. Power is work done over a specified period of time. When you multiply a torque by time (revs per minute) you get horsepower. In effect, HP is absolutely dependant on, and cannot exist without, torque. Torque can and does exist independantly of horsepower.
Straight Six engines have an inherent weakness that limits their horsepower capabilities. The long crankshaft has a tendancy to "twist" along it's length. Imagine making a model of a crankshaft out of rubber then grabbing both ends and twisting them in opposite directions. This is an exaggeration of what happens to all cranks when they are spinning with power being applied to their throws. In effect, the front end and back end of the crank are not always rotating at exactly the same speed.
How much effect this has is dependant on a combination of lot of design variables, like length, thickness, ductility of the material and the frequency of how often the power is being hammered against the rod throws. The problem with the I-6 is mostly the length. It breaks easily when the "torsional vibration" (twisting) gets into harmony with the other motion factors. Shortening a crank automatically reduces this problem, assuming other factors remain the same. This helps explain why a V6 or V8 will hold up to hi-RPM use better than an I-6.
Torque is a function of leverage and force. That's why it's measured in pound/feet (lb/ft). It's a certain force (pounds) applied over a lever arm (feet). The low revving V8 you mention COULD have a longer lever arm, in the "stroke" dimension which is the distance between the center of the crank and the place where the conn rod imparts it's force into the crank throw. (a 3-1/2" stroke is 0.29 foot)
Equally involved is the fact that a V8 engine has more power impulses for each revolution than a V6. To make this as simple as possible and ignoring for this example the fact that it's probably a 4 stroke engine, not a 2 stroke, lets assume for the moment that your V8 engine gets "pushed" 8 times by the force of piston & conn rod each time the crank rotates while the V6 only gets pushed 6 times. A V12 will get pushed 12 times, which explains why a 12 cylinder engine can make so much power.
A V4 only gets pushed 4 times, so it has to spin twice as fast to make as much power as a V8.
For the record; a Jaguar inline 6 makes enormous HP at high RPM, but not a lot of torque at low speed. A Caterpillar inline 6 makes an enormous amount of torque at low RPM but cannot do high RPM very well. The Jag has a short stroke while the Cat has a long stroke. The Jag also has a smaller piston and will have less fuel/air mix reacting on top of the piston, while the Cat will have a large bore and a lot of fuel/air reacting at each power stroke.
There are a hundred other factors that will determine why one engine makes low speed torque while another engine makes high speed horsepower, inlcuding the weight of the rotating assembly and the friction of internal components, but this is an overview of the basic differences.
Generally, the lower-revving engine has been designed to make a lot of torque at low RPM while the hi-revving engine has been designed to make HP at high RPM. It's not quite like comparing apples to oranges - it's more like comparing big, heavy Clydesdale draft horses (think; Budweiser beer wagon) to small, sleek, fast racing horses. You can have one or the other, but usually not both in the same animal. Of course, you can have an animal that is a reasonable compromise between the two, but it won't be as fast as the race horse and won't pull wagons like the draft horse.
Torque is the ability to twist a shaft, regardless of how long it takes to make a revolution. Power is work done over a specified period of time. When you multiply a torque by time (revs per minute) you get horsepower. In effect, HP is absolutely dependant on, and cannot exist without, torque. Torque can and does exist independantly of horsepower.
Straight Six engines have an inherent weakness that limits their horsepower capabilities. The long crankshaft has a tendancy to "twist" along it's length. Imagine making a model of a crankshaft out of rubber then grabbing both ends and twisting them in opposite directions. This is an exaggeration of what happens to all cranks when they are spinning with power being applied to their throws. In effect, the front end and back end of the crank are not always rotating at exactly the same speed.
How much effect this has is dependant on a combination of lot of design variables, like length, thickness, ductility of the material and the frequency of how often the power is being hammered against the rod throws. The problem with the I-6 is mostly the length. It breaks easily when the "torsional vibration" (twisting) gets into harmony with the other motion factors. Shortening a crank automatically reduces this problem, assuming other factors remain the same. This helps explain why a V6 or V8 will hold up to hi-RPM use better than an I-6.
Torque is a function of leverage and force. That's why it's measured in pound/feet (lb/ft). It's a certain force (pounds) applied over a lever arm (feet). The low revving V8 you mention COULD have a longer lever arm, in the "stroke" dimension which is the distance between the center of the crank and the place where the conn rod imparts it's force into the crank throw. (a 3-1/2" stroke is 0.29 foot)
Equally involved is the fact that a V8 engine has more power impulses for each revolution than a V6. To make this as simple as possible and ignoring for this example the fact that it's probably a 4 stroke engine, not a 2 stroke, lets assume for the moment that your V8 engine gets "pushed" 8 times by the force of piston & conn rod each time the crank rotates while the V6 only gets pushed 6 times. A V12 will get pushed 12 times, which explains why a 12 cylinder engine can make so much power.
A V4 only gets pushed 4 times, so it has to spin twice as fast to make as much power as a V8.
For the record; a Jaguar inline 6 makes enormous HP at high RPM, but not a lot of torque at low speed. A Caterpillar inline 6 makes an enormous amount of torque at low RPM but cannot do high RPM very well. The Jag has a short stroke while the Cat has a long stroke. The Jag also has a smaller piston and will have less fuel/air mix reacting on top of the piston, while the Cat will have a large bore and a lot of fuel/air reacting at each power stroke.
There are a hundred other factors that will determine why one engine makes low speed torque while another engine makes high speed horsepower, inlcuding the weight of the rotating assembly and the friction of internal components, but this is an overview of the basic differences.
Evil Result
05-14-2004, 11:27 AM
There are also the internal combustion process that depends on the compresssion, design of the cylinder head, and size of a bore compaired to the length of its stroke. All of which are effected by the weight that your moving and the gearing your using to get the power to the wheels....
So, if you have an engine reving to 10,000 RPM and you have a 2:1 gear ratio your input shaft whould be spinning at 10,000 RPM but output spinning at 5,000 but.... this also means that what ever torque you putting out at 10,000 RPM is multiplyed by 2. That means if your input shaft has 100 ft/lb of torque at 10,000 RPM your output shaft would being getting 200 ft/lb at 5,000 RPM that more than my smog dog V8 :)
i pulled those figures from an experement with Dyno2000 from an engine of 122 ci. i have a 318 ci in my car.
So, if you have an engine reving to 10,000 RPM and you have a 2:1 gear ratio your input shaft whould be spinning at 10,000 RPM but output spinning at 5,000 but.... this also means that what ever torque you putting out at 10,000 RPM is multiplyed by 2. That means if your input shaft has 100 ft/lb of torque at 10,000 RPM your output shaft would being getting 200 ft/lb at 5,000 RPM that more than my smog dog V8 :)
i pulled those figures from an experement with Dyno2000 from an engine of 122 ci. i have a 318 ci in my car.
59elcooldsuv
05-14-2004, 02:08 PM
There are also the internal combustion process that depends on the compresssion, design of the cylinder head, and size of a bore compaired to the length of its stroke.
So what do you think of Smokey Yunick's theory that long connecting rods improve combustion by increasing piston dwell at TDC, therefore delaying the increase in the volume of the combustion chamber so that reaction pressure builds up in the chamber before the piston moves down the bore?
I mean, aside from the obvious fact that longer rods reduce skirt-to-wall friction.
So what do you think of Smokey Yunick's theory that long connecting rods improve combustion by increasing piston dwell at TDC, therefore delaying the increase in the volume of the combustion chamber so that reaction pressure builds up in the chamber before the piston moves down the bore?
I mean, aside from the obvious fact that longer rods reduce skirt-to-wall friction.
SaabJohan
05-14-2004, 03:56 PM
Power from the engine is dependant on a few things, how much fuel that can be burned and how much energy this releases; and how large part of that energy that will be availible at the crankshaft.
The amount of energy released when burned with a certain amount of air is almost constant for all fuels with a few exceptions (like nitromethane), therefore the engine that can consume most air and transfer the released energy to the crankshaft will be most powerful.
The amount of air an engine consume is dependant on displacement, engine speed, volumetric efficiency and if the engine is 2 or 4-stroke (a 2-stroke consumes its displacement at 1 revolution and 4-stroke at 2 revolutions). Volumetric efficiency is defined as the amount of air an cylinder is filled with in relation to the amount of air that the cylinder could be filled with if the pressure was 1 atm. In racing engines NA engines can have peak values of over 130% and forced induction engines can have values much higher than that.
The problem when designing large engines is that the stroke will be long and therefore limit the speed of the engine, therefore more cylinders are prefered when designing a powerful engine. When designing for maximum efficiency the numbers of cylinders should be fewer as this decrease friction and heat losses.
Since power can de described as torque*angular velocity the more torque at a higher speed will give a more powerful engine. Sincve large engines can't rev that high they will instead give a higher torque. It should however be noted that engines with the same volume will give the same torque, unless they have forced inductions, for example the 3 litre F1 engines will give about the same amount of torque as any other 3 litre engine (that's a peak value of around 350 Nm, more or less). The F1 engine do however produce its torque at a very high engine speed and therfore it will have a high power output.
Since power = torque*angular velocity an engine can't produce any power without torque, power can however be produced without torque, but not in an engine. It should also be noted that torque can be produced without power, but then the angular velocity must be zero, that is when the torque has an equal amount of torque wotking in the reversed direction.
All materials deflect under load, the amount of deflection does however depend on the amount of material that is loaded and the module of elasticity of the material. In the case of crankshafts torsional dampers are also used.
This isn't the only problem with certain engine configurations, balance is another. For example an I6 will have a long crankshaft but otherwise perfect balance, the V6 will have a shorter crankshaft with will have balace issues. From the engines point of view an I6 engine, or a V12 is very good configurations but from the cars point of view a V6, or maybe a V10 can be a better choice. To find out which engine configuration is the best for a certain application it must be further investigated.
In any case, crankshaft deflection or vibration isn't that large issues with racing cars until the engines get very extreme.
What length the connecting rods should have have caused a lot of disagreement. Longer rods tend to be beneficial in high speed engines and especially if we want durability. In low speed engine a shorter rod tends to be better. This has to do with the piston motion, with a longer rod we decrease the peak acceleration at Top Dead Center and therefore decrease the load on the con rod. Since the con rod angle will be more narrow with a longer rod the side force on the piston will be smaller and therefore the friction loss will be smaller. The longer dwell time also allow more time for combustion which is good for a high speed engine. The downsides are that the longer combustion duration will cause a drop of power at low speeds and small bores, it also limits valve lift around TDC which can decrease volumetric efficiency and therfore power.
With power we can gear us to torque. Power, the mean power during the engine speeds that are used is what accelerate the car. For a certain car to reach a certain top speed a certain power is also needed, torque from that point of view is unimportant, it do however make the car more easy to drive.
The amount of energy released when burned with a certain amount of air is almost constant for all fuels with a few exceptions (like nitromethane), therefore the engine that can consume most air and transfer the released energy to the crankshaft will be most powerful.
The amount of air an engine consume is dependant on displacement, engine speed, volumetric efficiency and if the engine is 2 or 4-stroke (a 2-stroke consumes its displacement at 1 revolution and 4-stroke at 2 revolutions). Volumetric efficiency is defined as the amount of air an cylinder is filled with in relation to the amount of air that the cylinder could be filled with if the pressure was 1 atm. In racing engines NA engines can have peak values of over 130% and forced induction engines can have values much higher than that.
The problem when designing large engines is that the stroke will be long and therefore limit the speed of the engine, therefore more cylinders are prefered when designing a powerful engine. When designing for maximum efficiency the numbers of cylinders should be fewer as this decrease friction and heat losses.
Since power can de described as torque*angular velocity the more torque at a higher speed will give a more powerful engine. Sincve large engines can't rev that high they will instead give a higher torque. It should however be noted that engines with the same volume will give the same torque, unless they have forced inductions, for example the 3 litre F1 engines will give about the same amount of torque as any other 3 litre engine (that's a peak value of around 350 Nm, more or less). The F1 engine do however produce its torque at a very high engine speed and therfore it will have a high power output.
Since power = torque*angular velocity an engine can't produce any power without torque, power can however be produced without torque, but not in an engine. It should also be noted that torque can be produced without power, but then the angular velocity must be zero, that is when the torque has an equal amount of torque wotking in the reversed direction.
All materials deflect under load, the amount of deflection does however depend on the amount of material that is loaded and the module of elasticity of the material. In the case of crankshafts torsional dampers are also used.
This isn't the only problem with certain engine configurations, balance is another. For example an I6 will have a long crankshaft but otherwise perfect balance, the V6 will have a shorter crankshaft with will have balace issues. From the engines point of view an I6 engine, or a V12 is very good configurations but from the cars point of view a V6, or maybe a V10 can be a better choice. To find out which engine configuration is the best for a certain application it must be further investigated.
In any case, crankshaft deflection or vibration isn't that large issues with racing cars until the engines get very extreme.
What length the connecting rods should have have caused a lot of disagreement. Longer rods tend to be beneficial in high speed engines and especially if we want durability. In low speed engine a shorter rod tends to be better. This has to do with the piston motion, with a longer rod we decrease the peak acceleration at Top Dead Center and therefore decrease the load on the con rod. Since the con rod angle will be more narrow with a longer rod the side force on the piston will be smaller and therefore the friction loss will be smaller. The longer dwell time also allow more time for combustion which is good for a high speed engine. The downsides are that the longer combustion duration will cause a drop of power at low speeds and small bores, it also limits valve lift around TDC which can decrease volumetric efficiency and therfore power.
With power we can gear us to torque. Power, the mean power during the engine speeds that are used is what accelerate the car. For a certain car to reach a certain top speed a certain power is also needed, torque from that point of view is unimportant, it do however make the car more easy to drive.
quaddriver
05-14-2004, 11:37 PM
Ill answer the opening subject line....
for gasoline fueled, spark ignited engines:
hp = AP*VE*CR*CID*RPM/5252/150.8
ap = atmospheric pressure where you test
ve = volumetric efficiency at the RPM you are testing at
cr = compression ratio
CID = guess
you might notice that HP = Torque*rpm/5252,
therefore Torque = AP*VE*CR*CID/150.8
Or in simple terms, increase volumetric efficiency and spin it faster, and that and that alone determines ultimate hp.
for gasoline fueled, spark ignited engines:
hp = AP*VE*CR*CID*RPM/5252/150.8
ap = atmospheric pressure where you test
ve = volumetric efficiency at the RPM you are testing at
cr = compression ratio
CID = guess
you might notice that HP = Torque*rpm/5252,
therefore Torque = AP*VE*CR*CID/150.8
Or in simple terms, increase volumetric efficiency and spin it faster, and that and that alone determines ultimate hp.
59elcooldsuv
05-15-2004, 12:38 AM
Darn good answer, Dude.
SaabJohan
05-15-2004, 09:33 AM
You can't multiply with the compression ratio. This since the compression ratio will increase more than the power will when increasing it. The gain of a higher compression ratio will also be lower and lower the higher we go.
hp = volume*(rpm/(strokes per cycle/2))*VE*absolute pressure*(100/(287*temperature)/SAFR*energy content*efficiency/60000000*1,3596)
volume in cm^3
pressure in bar
VE in decimal form where 1 is 100%
efficiency same as VE
temperature in kelvin
energy content is about 44000000 J/kg for gasoline
SAFR is about 14.7 for gasoline
hp = volume*(rpm/(strokes per cycle/2))*VE*absolute pressure*(100/(287*temperature)/SAFR*energy content*efficiency/60000000*1,3596)
volume in cm^3
pressure in bar
VE in decimal form where 1 is 100%
efficiency same as VE
temperature in kelvin
energy content is about 44000000 J/kg for gasoline
SAFR is about 14.7 for gasoline
quaddriver
05-15-2004, 10:05 AM
You can't multiply with the compression ratio. This since the compression ratio will increase more than the power will when increasing it. The gain of a higher compression ratio will also be lower and lower the higher we go.
hp = volume*(rpm/(strokes per cycle/2))*VE*absolute pressure*(100/(287*temperature)/SAFR*energy content*efficiency/60000000*1,3596)
volume in cm^3
pressure in bar
VE in decimal form where 1 is 100%
efficiency same as VE
temperature in kelvin
energy content is about 44000000 J/kg for gasoline
SAFR is about 14.7 for gasoline
note that the two formulas are equivalent and produce the same numbers, its just that mine has only 1 normalizing constant (150.8) since my formula (actually its not mine, I swiped it) is for gas and SI motors it takes into account SAFR*energy content*efficiency/60000000*1,3596) then it replaces absolute pressure*(100/(287*temperature) with AP*CID*CR (and part of the constant)
but the point being, in both formulas, EVERYTING is static save VE and RPM
plus mine is a hell of a lot easier to measure and program if you wanted to graph it
hp = volume*(rpm/(strokes per cycle/2))*VE*absolute pressure*(100/(287*temperature)/SAFR*energy content*efficiency/60000000*1,3596)
volume in cm^3
pressure in bar
VE in decimal form where 1 is 100%
efficiency same as VE
temperature in kelvin
energy content is about 44000000 J/kg for gasoline
SAFR is about 14.7 for gasoline
note that the two formulas are equivalent and produce the same numbers, its just that mine has only 1 normalizing constant (150.8) since my formula (actually its not mine, I swiped it) is for gas and SI motors it takes into account SAFR*energy content*efficiency/60000000*1,3596) then it replaces absolute pressure*(100/(287*temperature) with AP*CID*CR (and part of the constant)
but the point being, in both formulas, EVERYTING is static save VE and RPM
plus mine is a hell of a lot easier to measure and program if you wanted to graph it
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