Torques, Diesel Vs Petrol etc help (For coursework)
Discussion
I seem to be here a fair bit recently for help with my CW 
For this I decided to use the Porsche Panamera Turbo
I've said basically petrols are more powerful than diesels. Not really sure why Petrols > Diesels (Google just wanted to tell me diesels were cheaper to run).
For power and torque, I don't even understand what the question wants for these. + The added benefit that I don't really understand torque. (Although thanks to Google I know its something about turning)
I'm not looking for anyone to answer this for me, but I would really appreciate some help on why petrol > diesel for performance and some help on the torque bit
For this I decided to use the Porsche Panamera Turbo
Question intro said:
Research a vehicle less than five years old with respect to engine / body construction and design
Question I'm kind of stuck on said:
Evaluate and justify the choice of engine used in terms of its layout, operation and performance. Answer should include reference to configuration, fuel type, power and torque
So I've done the bit about configuration (i.e not feasible to fit and in line 8 instead).I've said basically petrols are more powerful than diesels. Not really sure why Petrols > Diesels (Google just wanted to tell me diesels were cheaper to run).
For power and torque, I don't even understand what the question wants for these. + The added benefit that I don't really understand torque. (Although thanks to Google I know its something about turning)
I'm not looking for anyone to answer this for me, but I would really appreciate some help on why petrol > diesel for performance and some help on the torque bit
This should help, explains the relationship between power and torque:
http://www.carkeys.co.uk/features/technical/636.as...
http://www.carkeys.co.uk/features/technical/636.as...
OK, I'll have a go...
Torque is the force required to turn something: In an engine this is the crankshaft initially. The amount of force that can be exerted on the crankshaft is determined by the compression ratio in the cylinder - and that is why diesels have more torque than the same size petrol engine. Typically the compression ratio of a diesel is double that of a petrol engine.
Power is what keeps the crankshaft turning faster once the torque has overcome the resistance of the weight and aerodynamic drag of the vehicle.
In a nutshell, acceleration comes from torque, top-speed comes from power.
Makes sense?
Torque is the force required to turn something: In an engine this is the crankshaft initially. The amount of force that can be exerted on the crankshaft is determined by the compression ratio in the cylinder - and that is why diesels have more torque than the same size petrol engine. Typically the compression ratio of a diesel is double that of a petrol engine.
Power is what keeps the crankshaft turning faster once the torque has overcome the resistance of the weight and aerodynamic drag of the vehicle.
In a nutshell, acceleration comes from torque, top-speed comes from power.
Makes sense?
MGB Boy said:
I'm not looking for anyone to answer this for me, but I would really appreciate some help on why petrol > diesel for performance and some help on the torque bit
Torque is the twisting force. 'Work' is a force applied over distance. Power is work done over a given time.From this you can tell that Power is directly proportional to torque and rpm i.e. torque x RPM x constant = power
For a given weight you can generally get a higher power from a petrol engine than a diesel engine, hence it's better for performance.
snotrag said:
Odie said:
Why didnt you pick a petrol lawn mower or something interesting, everyone will pick supercars and normal boring stuff.
This.If you want to get good marks, do something interesting.
agree with the idea though- pick something other than a bog standard car, and you might have something more interesting to write about.
what about a boat? Somewhere on the net theres infformation about the largest internal combustion engine in the world. used on a mahoosive container ship. the engine is the size of a house, with pistons the size of rooms- that would make an interesting write up
Edited by Fordo on Thursday 15th April 16:28
Want to understand horsepower and torque? Read this: http://craig.backfire.ca/pages/autos/horsepower
Torque is the force applied, power is how often that torque is applied, or how much energy is supplied in a certain time period. Ie RPM x torque.
So 100lbft at 4000rpm will supply the same power as 200lbft at 2000rpm. Diesels make lots of torque but don't rev that highly, hence they dont produce much power.
Think of it as lots of little steps compared to one big step....
So 100lbft at 4000rpm will supply the same power as 200lbft at 2000rpm. Diesels make lots of torque but don't rev that highly, hence they dont produce much power.
Think of it as lots of little steps compared to one big step....
Edited by Adam205 on Thursday 15th April 16:32
MGB Boy said:
Starting to make some sense.
Anyone know why diesels are less powerful?
Diesels are genrally less powerfull as they rev lower. To make the same amount of power, they have to produce more torque as they dont rev as high. Power is just work done, a product of torque and revs.
Think of a pile of bricks at one end of the garden.
Torque is a measure of how many bricks you can carry at once. (i.e. a measure of force)
Power is how quickly you can move all of the bricks from one end of the garden to the other. (i.e. a measure of how much work can be achieved in a given amount of time.)
The above is as rough as badgers arrse, but I always thought it wasn't a bad stab at explaining the difference beteen the two.
Torque is a measure of how many bricks you can carry at once. (i.e. a measure of force)
Power is how quickly you can move all of the bricks from one end of the garden to the other. (i.e. a measure of how much work can be achieved in a given amount of time.)
The above is as rough as badgers arrse, but I always thought it wasn't a bad stab at explaining the difference beteen the two.
monthefish said:
Think of a pile of bricks at one end of the garden.
Torque is a measure of how many bricks you can carry at once. (i.e. a measure of force)
Power is how quickly you can move all of the bricks from one end of the garden to the other. (i.e. a measure of how much work can be achieved in a given amount of time.)
The above is as rough as badgers arrse, but I always thought it wasn't a bad stab at explaining the difference beteen the two.
I quite like that analogy - might use that in training Torque is a measure of how many bricks you can carry at once. (i.e. a measure of force)
Power is how quickly you can move all of the bricks from one end of the garden to the other. (i.e. a measure of how much work can be achieved in a given amount of time.)
The above is as rough as badgers arrse, but I always thought it wasn't a bad stab at explaining the difference beteen the two.
I think the term "more powerful" needs to be defined a bit more closely. If it means comparing naturally aspirated diesel and petrol engines of the same swept volume, then the fundamental difference is that diesel burns, petrol detonates (Wiki says "an exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it"). More chemistry orientated people will be able to tell you why, presumably because of the longer molecular chains.
Inherently, burning is a slower process than detonation, but technology has improved to allow smaller droplets of diesel to be created, which increases the area available for burning and so reduces the time it takes to burn any given quantity. Additionally, if you can pump higher pressure air into the cylinder, there are more oxygen molecules available and burning becomes faster and more thorough (hence turbo chargers). Hence, more fuel is burned, there are more cycles per second, and power increases.
One thing that is worth mentioning is that diesel appears to be more efficient than it really is, because it is denser than petrol. Petrol is around 737kg/m^3, diesel is around 850kg/m^3. i.e. around 15% more diesel per unit volume.
Inherently, burning is a slower process than detonation, but technology has improved to allow smaller droplets of diesel to be created, which increases the area available for burning and so reduces the time it takes to burn any given quantity. Additionally, if you can pump higher pressure air into the cylinder, there are more oxygen molecules available and burning becomes faster and more thorough (hence turbo chargers). Hence, more fuel is burned, there are more cycles per second, and power increases.
One thing that is worth mentioning is that diesel appears to be more efficient than it really is, because it is denser than petrol. Petrol is around 737kg/m^3, diesel is around 850kg/m^3. i.e. around 15% more diesel per unit volume.
RobCrezz said:
MGB Boy said:
Starting to make some sense.
Anyone know why diesels are less powerful?
Diesels are genrally less powerfull as they rev lower. To make the same amount of power, they have to produce more torque as they dont rev as high. Power is just work done, a product of torque and revs.
Think I should have enough to be going with for now, so thanks
MGB Boy said:
So I've done the bit about configuration (i.e not feasible to fit and in line 8 instead).
Even if it would fit a straight 8 is undesirable because of the long crank. The lack of rigidity in the long crank makes it susceptible to torsional oscillations which, if the engine is held at a speed which is harmonically related to the natural frequency, can cause fatigue and crank breakage. Also if the torque output is high enough the valve timing on the rear pots goes off...MGB Boy said:
Anyone know why diesels are less powerful?
Diesels always run very lean compared to petrols. Even at full throttle they are running about 50% lean of stoichiometric. If you try and put more fuel in the result is incomplete combustion and lots of black smoke, because with injection taking place only a few tens of degrees before TDC there simply isn't time for all the fuel to thoroughly vaporise and mix with the air. So you are never using the full potential of the air in the charge for combusting fuel, whereas with a petrol engine you do... the result is that a diesel engine will produce significantly less power at full pelt than a petrol with the same capacity and the same peak power revs.Zad said:
I think the term "more powerful" needs to be defined a bit more closely. If it means comparing naturally aspirated diesel and petrol engines of the same swept volume, then the fundamental difference is that diesel burns, petrol detonates (Wiki says "an exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it"). More chemistry orientated people will be able to tell you why, presumably because of the longer molecular chains.
No, no, petrol does not detonate - unless you have something wrong with the engine setup - in which case you tend to end up blowing holes in the pistons 
Indeed the combustion process in a diesel engine is notably closer to detonation than it is in a petrol - you get a BIG pressure spike from diesel combustion - hence the typical diesel "rattle" and (a lot of) the need for more robust construction.
In a petrol engine the fuel is drawn in with the air and there is plenty of time for it to vaporise and mix thoroughly with the air. When it is ignited a flame front spreads smoothly through the mixture from the spark plug.
In a diesel the fuel is injected not long before TDC and there is very little time for the vaporisation and mixing processes to take place. Also the combustion is not initiated from a well-defined single point, instead it is initiated by the bulk heat of the compressed charge. So basically not a lot happens until the fuel has vaporised and mixed sufficiently for combustion and then it all goes off at once. It still isn't "detonation" but it is a much sharper process than combustion in a petrol engine.
Polarbert said:
HellDiver said:
MGB Boy said:
Anyone know why diesels are less powerful?
They aren't. It's just a different.How the f
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