Why are diesel engines considered more reliable than petrol engines.

Refueler said:
Just like to correct one bit ... posts here mention Petrol Engines usually based on car ... diesels not.

mmmmmm Perkins 4 series engines powered London Cabbies, 50's school buses etc. etc. as well as industrial stuff. Perkins were not only to do it. BMC - based on car engines like Marina / Montegos etc. Whole batch actually ...

Funny thing is many petrol engines were a lot smaller HP than the diesel replacements ... Vire 6 replaced often by Petter 12's etc. Never saw a car with small Vire or Dolphin engine ....

:D:rolleyes:
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Yes all very true but knowing you are into fuel in a big way I am astounded that you and everyone else on here seem to be ignoring the volatility of gasoline (petrol) vapour.
The biggest horror on a boat is the possibility of FIRE. I am very careful with the very small amount of 2 stroke petrol I carry on board for my outboard. I certainly would not want a petrol engine on my boat. Although I recognise that some powerboats carry large tanks of the stuff on board there is absolutely no compelling reason to carry it on a sailboat!
 
but the OP was about reliability not volatility of the fuel. I agree that na diesel fueled engine is safer than a petrol fueled engine in event of leaks / fire etc.

I can well remember my old Vire 6VRE and when seals went in crankcase - having to flood it to get it to start. Cabin would stink of gasoline.

Outboard fuel ? I carry a can of it in transom locker, always have and not really bothered about it. I make sure it's got plenty of gear between it and exhaust hose on other side of locker (my locker spans across the whole transom), plus cap on tight. Nowhere else really to stow it.

I do feel that my old Vire before it blew it's seals gave a very good wide speed range, with the coil ignition I fitted - it started easily and was quieter .. smoother than any diesel I've had. But still I wouldn't go back to one ... except I'd love one with associated running gear for my little weekender out here !!
 
michael_w said:
I thought it was because the explosion within a diesel engine is much more violent, therefore the engine has to be built stronger to withstand the stresses.

Also, you don't have all that high tension stuff to go wrong.
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You have hit the nail on the head PLUS most diesels run at relatively low speeds and so the piston speeds etc are lower than petrol engines. My 206 HDI runs at 2500 rpm at 70ish as opposed to 3500 for my Micra. If one was to work out the distance travelled by the piston, converted from reciprocal to linear, one would find that the piston has travelled slower and less distance in a diesel as opposed to a petrol. The calculations would also work on the bearings etc. So although they are built tougher than petrol engines they do less "work"!
 
The rev difference is more important than it first looks, because the loads increase as a square of the RPM, one of the reasons that fitting a turbocharger is a 'kind' way of increasing power, as opposed to increasing the revs.
Having rebuilt several engines of both types, the diesels are much tougher in construction.
As for mileage, trucks can go much more than 300K, more like 1 million. All my diesels are 'old' tech, no electronics, but I hear that the modern car diesels are not a lot more reliable than the petrol equivalents.
A
 
Diesels do have 6 negatives which no-one seems to have mentioned:
a) with the exception of a few 'sensible' hand-startable marine brands, they require a healthier battery than the equivalent sized petrol engines, and b) they tend to be heavier on starter motors.
c) diesel fuel tends to grow 'bugs' in warmer climes.
d) diesel fuel is prone to frothing in heavy weather, and unless precautions are taken (several methods), this aeriation can stop the engine, the fuel delivery system of which will then require bleeding.
e) Diesel lube oil is more expensive than the petrol equivalent, and requires changing more often.
f) Turbo-diesels are an accident waiting to happen. If the turbo seals should fail, there will be nothing to stop the engine revving itself to destruction, whilst the engine oil is sucked out of the sump and burnt as fuel.

Having said this, all-in-all normally aspirated diesels are a better choice for marine applications, unless raw speed is a requirement.
 
electrosys said:
Diesels do have 6 negatives which no-one seems to have mentioned:
a) with the exception of a few 'sensible' hand-startable marine brands, they require a healthier battery than the equivalent sized petrol engines, and b) they tend to be heavier on starter motors.
c) diesel fuel tends to grow 'bugs' in warmer climes.
d) diesel fuel is prone to frothing in heavy weather, and unless precautions are taken (several methods), this aeriation can stop the engine, the fuel delivery system of which will then require bleeding.
e) Diesel lube oil is more expensive than the petrol equivalent, and requires changing more often.
f) Turbo-diesels are an accident waiting to happen. If the turbo seals should fail, there will be nothing to stop the engine revving itself to destruction, whilst the engine oil is sucked out of the sump and burnt as fuel.

Having said this, all-in-all normally aspirated diesels are a better choice for marine applications, unless raw speed is a requirement.
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Turbo seals failed on a colleagues work car. Car was written of by the leasing company as being beyond economic repair
 
electrosys said:
Diesels do have 6 negatives which no-one seems to have mentioned:
a) with the exception of a few 'sensible' hand-startable marine brands, they require a healthier battery than the equivalent sized petrol engines, and b) they tend to be heavier on starter motors.
c) diesel fuel tends to grow 'bugs' in warmer climes.
d) diesel fuel is prone to frothing in heavy weather, and unless precautions are taken (several methods), this aeriation can stop the engine, the fuel delivery system of which will then require bleeding.
e) Diesel lube oil is more expensive than the petrol equivalent, and requires changing more often.
f) Turbo-diesels are an accident waiting to happen. If the turbo seals should fail, there will be nothing to stop the engine revving itself to destruction, whilst the engine oil is sucked out of the sump and burnt as fuel.

Having said this, all-in-all normally aspirated diesels are a better choice for marine applications, unless raw speed is a requirement.
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Hmm, Ive worked in the middle east and africa for the majority of my working life as head of maintenance for various oil majors. I have looked after countless diesel engines from humble single cylinder ones to massive V 16 ones, both four stroke and turbo, supercharged two stroke ones. I have NEVER had to deal with the so called diesel bug, I have only ever changed one starter motor (on a spanish assembled land rover where they used some spanish **** electricals). I have never had a problem with frothing diesel fuel (I looked after offshore stuff as well) I do not recognise that diesel lube oil is more expensive than petrol stuff, in fact we used to use a single oil by shell or texaco which was made to be used in either engine. I NEVER saw a single instance of a turbo charged engine do what you describe, on GM 2 stroke engines flaps were fitted on the intake so that if the SUPERCHARGER seals went then the flap could be used to stop the engine. I also saw intake flaps fitted to stop four stroke engines, BUT, this was in case gas became available to let the engine run away (think oil drill rigs and production facilities where the possibility of gas leaks could come about)
So working on repairing and maintaining literally hundreds of diesel engines for nigh on 40 years and I never personally came across any of the scenarios that you describe! See my previous post as to why diesels last longer than petrol ones.
Stu
 
skipper_stu said:
You have hit the nail on the head PLUS most diesels run at relatively low speeds and so the piston speeds etc are lower than petrol engines. My 206 HDI runs at 2500 rpm at 70ish as opposed to 3500 for my Micra. If one was to work out the distance travelled by the piston, converted from reciprocal to linear, one would find that the piston has travelled slower and less distance in a diesel as opposed to a petrol. The calculations would also work on the bearings etc. So although they are built tougher than petrol engines they do less "work"!
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BUT the stroke lengths to get the compression are far longer, therefore in distance travelled by the piston, a petrol and diesel must be ramarkably similar
 
Given that, as with most things in this universe, things get worse proportional to the square of the speed/load/area then a diesel running at an average of say 2500 rpm vs a petrol running at an average of say 5000 rpm the petrol engine is experiencing 4 times as much wear. ( or something like that )

Diesels are happier running at lower overall temperatures and that's a big corrosion / wear influence.
 
Chris_Robb said:
BUT the stroke lengths to get the compression are far longer, therefore in distance travelled by the piston, a petrol and diesel must be ramarkably similar
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Ermm, sorry, wrong, the combustion chambers are smaller, most diesels have flat heads with the chamber in the piston OR a chamber as a pocket with the injector sticking in. Example, VP MD22, converted BL petrol engine, bore and stroke the same as the petrol engine.
Stu
 
Well in theory and in a marine environment I would certainly say an old Diesel would be preferable to petrol.
However, if you look at the old Volvo Penta V8 petrols which are effectively Small Block Chevy engines then you can't use the simplicity card.

My Shetland has a V8 Petrol with a Rochester carb and honestly it doesn't come much more simple than that.

I would agree that ignition circuits don't like damp sea air and that can be a negetive and of course you need to run bilge fans to clear petrol vapour before running so potentially there is a safety issue too.

Regarding reliability, well i'd certainly conceed an old relatively low-output diesel (in relation to its capacity and weight) are extremely reliable old beasts.

However, some of the modern, light, high-output diesels have a much more variable record.

When you start chasing big HP, light weight, and lots of complexity to achieve this along with reducing emissions then there is a trade-off and that is reliability.

Not wishing to single out VP but some of their small High Output diesels have had atleast their fair share of problems.

So I would say the safety issues remain the same and electrical reliability the same but I certainly wouldn't say modern diesels are the same as the 'Run n Forget' diesels of old.
 
I am quite surprised Skipper Stu has never come across the diesel bug. It is most certainly around, and I had it on one of my boats some time ago. At least, the black tarry compound the fouled up the fuel system fairly thoroughly until I got the tank properly cleaned out looked exactly like the stuff commonly described as the 'bug'. I do think that yacht auxiliaries are more prone to it when they are left standing with small amounts of water in the fuel tanks sometimes for weeks on end undisturbed, and IIRC Refueller reckons the microbes that cause it are always present in diesel fuel.

I too never ever heard of frothing causing problems at sea: more likely a nearly empty tank being bounced around and uncovering the outlet or pick up pipe.

I have twice had turbo seals fail on high mileage road vehicles. Neither caused the engine to 'self destruct'. One simply lost power and smoked if you tried to rev it, the other continued to work, but crankcase pressure started forcing oil up the breather and caused bad smoking again. It ran away for about 20 seconds on lube oil in the inlet - at which point the driver stopped and had the vehicle towed in. The engine was undamaged.

IMHO the only reason they could be 'heavier on starter motors' would be a poorly maintained engine that required extended cranking to start it, causing the starter motor to overheat . But Petrol engines starters are just the same, and will burn out just as easily.

As to the OP question about reliability - both types are reliable only IF PROPERLY MAINTAINED.

An abused diesel will continue to run - sort of - longer than a petrol engine. But repairs will be considerably more expensive.

How many of use LPG for cooking in our boats? I would guess, ther vast majority. LPG and petrol are both highly flammable, and explosive when present in air in quite small concentrations, and present pretty well the same level of risk. So I personally dont buy that one, and happily run a petrol engine as an auxiliary. 100% precautions need to be, and on my boat are, taken with both the gas and petrol supplies.
 
skipper_stu said:
Example, VP MD22, converted BL petrol engine, bore and stroke the same as the petrol engine.
Stu
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Perhaps it was chosen because it had a long stroke (ie: was undersquare) whilst most modern manufacturers of petrol engines have been producing shorter stroke (oversquare) engines for some time.
Of course the piston speeds of oversquare, higher revving, petrol engines will be higher but whilst the forces to change the direction of travel of the pistons at each end of the stroke may seem to be greater, the (admitedly wider) pistons in an oversquare petrol engine will be lighter and less massive than those required to withstand compression ratios in the 26:1 range of diesel engines rather than the typically 12:1 of modern high compression(?) petrol engines.
 
Lakesailor said:
Perhaps it was chosen because it had a long stroke (ie: was undersquare) whilst most modern manufacturers of petrol engines have been producing shorter stroke (oversquare) engines for some time.
Of course the piston speeds of oversquare, higher revving, petrol engines will be higher but whilst the forces to change the direction of travel of the pistons at each end of the stroke may seem to be greater, the (admitedly wider) pistons in an oversquare petrol engine will be lighter and less massive than those required to withstand compression ratios in the 26:1 range of diesel engines rather than the typically 12:1 of modern high compression(?) petrol engines.
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The MD22 has a ratio of between 17 and 18.1 - 1 depending on spec, see here http://www.beneteau-owners-association.org.uk/md22l_3.pdf about two thirds down the page.
Stu
PS am I starting to sound like VicS?
 
Well they can run on expired fish & chippie's cooking oil

or anything much else 28-35 sec oils that you can thrown at them.

Reliable ... yes ... as long as you don't squash the piston/conrod/bend_the_crankshaft (by backflooding) ... or get anything microscule past the final filter into the high pressure injector pump ... or air or bugs clogging the fuel system .....

but apart from that if you press the starter on a no glow plug engine and the starter motor engages and you get an IMMEDIATE thunk-thunk-thunk then you know that you are in the realm of the blessed.

Isn't it time that both PBO and YM got out of their repetitive gardening mode ie plant daffodil bulbs in the autumn and got into a research mode: Why is it that small diesel engines get progressively harder to start and what causes this progressiveness ......

If you go back for the past 30 years in both magazines there is nothing ie NOTHING that they have pontificated on, that addresses the question of why returning exhausted water from improperly designed/blocked exhaust systems finally sinks small diesels .... and there is not a single addressing of the the question "My engine doesn't start" and the boatyard says "(Sharp intake of breath) ... YIP - You need a new engine - What you've got can't be rebuilt - Well it can but it will cost you twice as much as a new engine ..." .... and that is RUBBISH!

How many Yanmars, Betas, Nannis etc etc etc have been sunk and are used as tackle when they could have been rebuilt?????
 
skipper_stu said:
The MD22 has a ratio of between 17 and 18.1 - 1 depending on spec, see here http://www.beneteau-owners-association.org.uk/md22l_3.pdf about two thirds down the page.
Stu
PS am I starting to sound like VicS?
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Yes. I was suggesting compression ratios at the top end for both types of engine. "A petrol engine compresses at a ratio of 8:1 to 13:1, while a diesel engine compresses at a ratio of 14:1 to as high as 26:1."
Yes You are starting to sound a bit like VicS. But that's not a bad thing.
 
Lakesailor said:
Yes. I was suggesting compression ratios at the top end for both types of engine. "A petrol engine compresses at a ratio of 8:1 to 13:1, while a diesel engine compresses at a ratio of 14:1 to as high as 26:1."
Yes You are starting to sound a bit like VicS. But that's not a bad thing.
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Never mind this tech stuff how about some tits to brighten us up?
 
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