Thread: Eny meny miney mo which diesel motors the go

Do TD5's need valve adjustment at regular intervals?

Td5's use hydraulic lifters so don't need adjustment. I have seen some wear badly though, probably from lack of oil changes.

Originally Posted by Chucaro

Going by my experience with the service vehicles that we used to operate on the rice farms servicing John Deere tractors you are miles wrong.
We have Ford F100 with Perkins 4 cyl diesel engines and some V8. The Perkins lasted 4 times more under the same conditions.
We have Peugeot 404 utes petrol and diesel, the diesel lasted 3 times more than the petrol engines.
We have Ford 250 utes fitted with John Deere and Perkins 6 cyl diesel engines. The John Deere and Perkins lasted 5 times or more that the V8 Ford petrol engines uden the same conditions.
Just drive a petrol ute day in day out on second gear on a clay muddy road for 3 hours with a ton of load both vehicles and you soon will fid out

So you get a petrol and a diesel engine and both of them have cold oil in the sump and when you start them up the petrol with wear but the diesel won't?.There is a million and one things that decides how long any engine will last,the idea that a diesel will outlast a petrol is wrong but it is the general mindset that diesels never wear out. Pat

Diesels do outlast petrols, well generally anyway. Its got nothing to do with any whiteman magic engineering principals or mechanical differences between the 2 types of engines its to do with the history of the engines.

lets just skip the whole rudolph diesel and his peanut oil fired engines and go straight to my favorite engines the 2.25 series landrover diesel and petrol engine. why? because the 2 are so close to identical that in many respects they are. Crank, block, rods, pistons, flywheel, cooling system and valve gear are identical, they are so close that if you took a diesel engine, plugged the glow plug holes, tapped the injector holes, did some machining to the combustion chamber, added a carby + sparkplugs, hoicked the injector pump dropped in a dizzy and a coil and viola, one ex diesel come petrol 2.25 series engine.

now you'd think that with so much in common that the engines would last the same but they dont.

and heres one example why.

hands up all of you series diesel owners, what happens on a cold day just after you start it and you try to make it do some work?

It stalls

This happens really easily becasue its too cold, the air that gets compressed to ignite the diesel is cooled by the metal of the engine so the ignition is led by a combination of the glow plug and the compression. Invariabley this means it fires late so its not making as much power and most of the power thats being made is being used to compress the air for the next ignition (recall the white smoke of start up that gradually fades as the engine warms up).

What do most people do about this? they let it warm up, you've got no choice its an old diesel, if it aint warm it wont work.

The petrol engine on the other hand provided its in tune will start and run, you may need to choke it but because its lower compression once its running its pretty much good to go.

so even tho the engines are identical (well more or less) due to the nature of the beast the diesel generally gets treated nicer than the petrol (well the older ones that started the whole diesels last longer thing anyway)

the other side of it is this, Naturally aspirated CC per Naturally aspirated CC Diesel isnt as agressive as petrol, the petrol version of the 2.25 has it all over the diesel by about 15% across the board sure it uses more fuel doing it but thats a fuel density issue. The more power you ask for the faster you wear the engine out The petrol's got more power to give and it generally gets asked to give it.

Just on that consideration in the real world the diesel will outlast a petrol engine. However in lab conditions if you made all things equal both engines would wear out at the same rate.


some other things that make a difference.

diesel fuel is a lubricant, if you diluted the engine oil with a liter of diesel you'd do less damage than if you tipped in a liter of petrol (youd also be less likely to blow your self up from hot combusion gasses igniting the engine oil)

Diesels tend to take more turns to fire so will often develop oil pressure while still cranking on the starter motor

The diesel has a govenor on it so if you floor it with no load on it it will only hit a given RPM the petrol engine will rev up untill either the piston speed exceeds the combustion speed, an engine dynamic acts as a limiter or it throws something out.

diesels arent so fussy on fuel air ratios, If they run too rich they speed up and the govenor winds it back if they run too lean well they almost always run lean if they do carbon up they tend to run a little better due to the increased compression giving a better light off of the fuel charge unless a valve sticks in which case that pot makes no power untill it clears itself or you do something to fix it. If a petrol carbons up from rich running it can cause pinging and hot spot run on, Run it lean and you can burn out the valves and pit the pistons.

Timings important on both more so on the diesel as if its too far wrong it just wont run but the petrol will struggle on Im unconvinced as to which is going to cause the most damage by running with off timing.

it should also be indicated that for this its assumed that there are no defects in the metals or the making of the engines. Once you start including these things all bets are off. And this is ONLY about the old school engines not these new ally headed things.

Originally Posted by Blknight.aus

the other side of it is this, Naturally aspirated CC per Naturally aspirated CC Diesel isnt as agressive as petrol, the petrol version of the 2.25 has it all over the diesel by about 15% across the board sure it uses more fuel doing it but thats a fuel density issue.

The energy density difference between diesel and petrol isn't the only reason they use less fuel.
A litre of diesel has ~39MJ worth of energy (10.8 kwH)
A litre of petrol has ~35MJ worth of energy (9.7 kwH)
That's only 11% more energy.

The rest of the fuel consumption difference comes from the higher compression (roughly double) which gives you double the expansion to get energy from the burnt fuel and the lack of a throttle plate which saves whole kilowatts in pumping losses at part throttle.

The slower burn speed also makes more usable torque and power at lower rpm where fuel economy is better.

Originally Posted by Dougal

The energy density difference between diesel and petrol isn't the only reason they use less fuel.
A litre of diesel has ~39MJ worth of energy (10.8 kwH)
A litre of petrol has ~35MJ worth of energy (9.7 kwH)
That's only 11% more energy.

The rest of the fuel consumption difference comes from the higher compression (roughly double) which gives you double the expansion to get energy from the burnt fuel and the lack of a throttle plate which saves whole kilowatts in pumping losses at part throttle.

The slower burn speed also makes more usable torque and power at lower rpm where fuel economy is better.

As well as being able to keep feeding the fire so to speak with a diesel, keeps the torque levels up and combustion temps down.

With a 300Tdi with ally head for example, the only difference between what Daves put there and now is that the 300Tdi will warm up quicker due partly to the ally head and cooling system design but also cranks over less to start due to low compression until it is on boost (in comparison to a NA diesel).

This low comp start up also means that it has little load on parts before you get oil pressure and with a higher capacity oil pump and better designed galleries ect that takes a lot less time.

Once you have oil presure, all bets are off, doesnt matter what engine you have, a cold engine with oil presure is a cold engine with oil presure and a 300Tdi wont produce much boost until it is a little warm anyway.

Regarding the compression, I haven't seen a direct injection diesel with a compression ratio higher than 18.something. It's the indirect injection diesels that get up to 22:1.

My work car is a 2.2 direct injection non turbo (needs a turbo) and it's still 18.0:1

The fuel density issue is a double headed beast.

the first part of the deal is that you get more "particles" of fuel per liter when you buy diesel as opposed to petrol and thats not getting into the fact that diesel has fewer component pieces to it than petrol (think of the addative packages) thats where you get the majority of the difference in the 35 vs 39Mj of energy in the 2 fuels.. (and from hazy memory I think that diesel is nearer 9MJ better of than petrol as I used to remember the difference as the sum of the compression numbers for a 2.25 but that might also be using older fuel specs)

the second part is how the fuel burns off.

heres a lateral equivalence experiment for you.

buy a 30cm cubed block of untreaded pine, split it up make some kindling out of it and some bigger stuff and light it up now do the same with some australian hard wood.

you should notice

before you burnt it all the block of pine weighed less than the hardwood.
if you split it up fine enough the pine is lightable without the use of paper or petrol.
if you want the hardwood to light up you need to split it up a lot finer than pine
the pine once lit develops the fire faster
the pine developed a good heat faster than the hardwood but doesnt get as hot (assuming that you built both fires in the same manner)
the redwood burnt longer.
the redwood left more ash and coals than the pine did.