Torque of diesel vs Petrol

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Originally Posted by artme

Thanks Bob!, A good read. Must read again so it sinks even deeper into the realms of cognitive functtion.

Diesel contains more enery per litre or per kilo than petrol?? Which is it??

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Diesel provides 0.2% J/kg less than petrol but since is 12% denser it has more J/L.

Extracted From WikiP

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As of 2010, the density of petroleum diesel is about 0.832 kg/L, about 12% more than ethanol-free petrol (gasoline), which has a density of about 0.745 kg/L . About 86.1% of the fuel mass is carbon, and when burned, it offers a net heating value of 43.1 MJ/kg as opposed to 43.2 MJ/kg for gasoline. However, due to the higher density, diesel offers a higher volumetric energy density at 35.86 MJ/L vs. 32.18 MJ/L for gasoline, some 11% higher, which should be considered when comparing the fuel efficiency by volume. The CO2 emissions from diesel are 73.25 g/MJ, just slightly lower than for gasoline at 73.38 g/MJ.[8] Diesel is generally simpler to refine from petroleum than gasoline, and contains hydrocarbons having a boiling point in the range of 180-360°C (360-680°F). ".

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Some of those articles are quite good.

Unfortunately to really properly understand the balancing act mechanical engineers perform when trying to design a good engine takes years of study.

A _good_ engineer understands exactly the parameters we have to work within. The trick is matching these to customer expectations. The two things that have changed since 1900 are supporting technologies like filters, electronics, metalurgy etc, and customer expectations.

If you took a 2011 car back to 1930 you simply couldn't sell it. Even if it were affordable the fuels filters and oils available then would make ownership a nightmare. Sportier models wouldn't handle the roads (we're getting back to that situation hence the rise of "suvs"). Imagine decoking a double over head cam motor every 10,000 miles.

Diesels are somewhat more thermally efficient than petrol engines because the higher compression means smaller surface area and hence less heat loss through the combustion chamber than petrol.

As a rule the work done in an internal combustion engine is the gas expansion in the first 29 degrees after top dead center bearing on the piston crown. Everything else is losses. Friction, heat loss through cylinder walls and exhaust, fuel burned after that angle, all the work to get to that point. Because the flame propogation is a constant rate for a given fuel you can use gas velocities in the chamber to speed it up or you can slow the revs so that 29 degrees of crank rotation happens over a longer time. I should qualify that statment for the pedantic that drop size, temperature and other factors afftect burn time, but if your focusing on design after the inlet closes than they are largely out of your control then.

It's incredibly complicated and to be honest I've only ever encountered a handful of people in my life who really understand combustion chambers. I know enough to know I don't and I've met an awful lot of people who think they do and really don't.

I am a motor mechanic by trade and mechanical engineer by profession. I worked briefly in the automotive industry a long time ago. Glimpsing the hard stuff is scary, the realisation of just how little you understand.

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Originally Posted by damian

If you took a 2011 car back to 1930 you simply couldn't sell it. Even if it were affordable the fuels filters and oils available then would make ownership a nightmare. Sportier models wouldn't handle the roads (we're getting back to that situation hence the rise of "suvs"). Imagine decoking a double over head cam motor every 10,000 miles.

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I asked my 87 year old uncle who was a mechanic in the air force in WWII what are the most profound changes he has seen in his lifetime. After giving it a lot of thought he said, "the quality of lube oils and the length of womens skirts"

He reckons the quality of aircraft lubes the Americans had was a significant reason for the success of their heavy vehicle tech. Combined with engine manufacturing this adds amazing longevity to the latest quality engines. I see the latest diesel van from Fiat recommends oil changes only every 45,000 kms.

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Originally Posted by BobL

I asked my 87 year old uncle who was a mechanic in the air force in WWII what are the most profound changes he has seen in his lifetime. After giving it a lot of thought he said, "the quality of lube oils and the length of womens skirts"

He reckons the quality of aircraft lubes the Americans had was a significant reason for the success of their heavy vehicle tech. Combined with engine manufacturing this adds amazing longevity to the latest quality engines. I see the latest diesel van from Fiat recommends oil changes only every 45,000 kms.

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remember de-coking? (oops Damian did)
Cheers,
Jim

OK. Given all the above info - thanx Bob and Damien - let me ask another question or two.

From what I understand (( and Ignorance is my vantage point:B) Diesel engines have a longer stroke, in general terms than petrol engines. This in turn produces the higher torque diesel engines are known for.??

Now, given the extra energy contained in diesel fuel, and the slower rate of burning, does this mean that the combustion of the diesel fuel has more time to push the piston over the longer distance - with a little more force - and thereby add to the extra torque,?????

Getting back to Damien's point about vehicles in the 30's made me remember a conversation my brother and I had.
We reckoned that if Burke And Wills, Blaxland Wentworth and Lawson and all those other fellas in that era had a deisel Landcruiser Australia would have been "discovered" so much earlier than what history tells us :U

Sort of.

There have been a number of engines built over the years in both diesel and petrol variants but otherwise the same. The ferguson tractor was one I happen to be familiar with, used the 2 liter standard/triumph 4 cyl motor. Even with teh same stroke and bore the diesel gives more torque because of the higher pressures they work at and tend to give less power because the slower burn limits mazimum revs.

Long stroke is like gearing the motor down, but because on a longer stroke the forces are bigger revs are limited.

It's complicated. For example the rod length to stroke ratio has an impact on both wear rates and maximum revs because it changes the side thrust on the piston.

Torque is a function of how much force the gas is applying to the piston crown in that angle of rotation and the leverage which is increased by the longer stroke and by gearing down. Generally diesels are designed for high torque and lower revs because the system, the fuel and the cycle, lends itself to that.

Lately diesels have been designed for higher and higher revs and apparently audi is trying to build a diesel lemans winner. It's not the natural choice for that sort of work but there are marketing incentives to build "green" cars and demonstrate leading diesel technology.

Ah yes the old four cylinder diesel Fergies! Caused more than enough headaches and yet the petrol version was as tough and reliable as they come. Sir Edmund Hillary used them in the Antarctic because of this,

I had a TR3A many many years ago. They didn't beef them up enough when they did the diesel conversion.

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Originally Posted by chambezio

Getting back to Damien's point about vehicles in the 30's made me remember a conversation my brother and I had.
We reckoned that if Burke And Wills, Blaxland Wentworth and Lawson and all those other fellas in that era had a deisel Landcruiser Australia would have been "discovered" so much earlier than what history tells us :U

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Or quicker again if they had had a helicopter, or a hercules.

Of course if they'd been using cruisers they wouldn't have broken down but they still would have had to get out and walk at the first sign of big rocks, deep fords, deep mud...

They are handy on a dirt road but I can show you plenty of places a range rover or a defender will trundle along that will confound a cruiser, or most other japanese 4b's.

Oddly enough the forester is surprisingly good off road. The electric thingies (technical term) that lock the wheels make it quite capable with 2 wheels hung up and getting traction on slippery surfaces. I wouldn't go banging around the Jardine though...