ELI5

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Re: ELI5

Postby Johnny Byrne's Boots on Fri Mar 17, 2017 2:52 pm

claretandblue82 wrote:
I'd like to see who much of this you could say to a 5 year old before they get distracted by something else.

...............................


Fair point, but a five year old who asked that specific question just might last to halfway :wink:
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Re: ELI5

Postby White Goodman on Fri Mar 17, 2017 4:03 pm

Johnny Byrne's Boots wrote:Diesel fuel contains more energy than the same amount of petrol. The problem is it burns more slowly than petrol, so in a given time you can extract more energy from petrol than diesel but you use more of it. As the revs rise the petrol engine will carry on producing more power, the diesel will reach a point where the previous charge of fuel has not finished burning before the next one comes along. You can see this as plumes of soot (unburnt fuel) from the exhaust.

Each power stroke of the engine turns the crankshaft with a given force, greater in a diesel engine all other things being equal. The engine doesn't drive the wheels directly, there's a gearbox in the way. This matches the engine revs to the roadspeed. A diesel engine, because it produces more torque, or turning force, will be higher geared than the petrol version. 70 mph in the diesel may happen at 2000 rpm, 2500 in the petrol.

Some sums. Power equals torque x revs. Torque in this case can be defined as 'how hard the piston turns (or pushes) the crankshaft' and power as 'how often it turns it in a given time'. A 'push' of one hundred units delivered one hundred times in a second will produce twice the power of the same push delivered fifty times.

When you get to the upper reaches of the rev ranges, the diesel begins to lose efficiency at around 4000 rpm, it just can't burn the fuel quickly enough.The petrol engine can go to 7 or 8 thousand rpm, and although it produces less force with each push on the piston it can produce more of them in a second and produces more power.

As 0-60 times take the engine to the red line, the petrol engine spends more time than the diesel producing more power.

'Excess' power produces the acceleration. If a car needs 50 bhp to maintain a steady speed overcoming air and road resistance, when you put your foot down and produce more power the car accelerates until the power produced is no longer greater than the power needed to maintain current speed. Air resistance is the major factor and increases as the square of the speed. It takes four time the power to do 100 than it does to do 50.


Great explanation :thup:
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Re: ELI5

Postby White Goodman on Fri Mar 17, 2017 4:06 pm

Johnny Byrne's Boots wrote: It takes four time the power to do 100 than it does to do 50.


Is that why the upgraded Bugatti Veyron could only do about 10 miles an hour more than the standard version despite having an extra 200hp??
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Re: ELI5

Postby Johnny Byrne's Boots on Fri Mar 17, 2017 6:17 pm

^^^^ Pretty much.

A family car with 120 bhp will do nigh on 120/130 mph. A 600 bhp Lambo with five times the power will do less than twice that.

Huge tyres and downforce producing aero costs drag. Lots of it.


Random fact. According to F1 drivers, just lifting your foot off the accelerator and not using the brakes slows an F1 car at about the same rate as an emergency stop in your road car. All that downforce induces lots and lots of drag.
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