Horsepower, revs and engine size

[LondonRascal]

Engineers, wizards or just wise people...

This has been on my mind for some time and I can’t really figure it out:

 

You have two boat engines one produces 80HP and the other 85HP. 

 

One engine and produces 80HP at 2,500RPM and is a 4.085 litre engine.

 

The other engine produces 85HP at 3,600RPM and is a 3.168 litre engine.

 

So the question I have is - the lower horse power producing engine is actually a larger engine as it’s displacement it just over 4 litres, but it revs a lot lower at 2,500RPM.  The higher Horse Power producing engine is smaller but revs a lot higher but gives you an extra 5HP.

 

Would the higher HP but smaller displacement engine give better fuel economy, or would a larger displacement engine revving slower give you more fuel economy?

[grendel]

with cars I can say that larger lower horsepower engines give better economy than smaller higher horsepower, take the example of 2 Volvos I have owned, one was a 2.0 litre turbo, on a smaller car, this averaged at 32mpg (if I was careful), my current, bigger- heavier Volvo with a 2.5 litre engine (no turbo) gives me an average of 36mpg on the same run (as most of the time it is doing less work to keep me moving)  The actual BHP for the two vehicles is practically identical (that is pulling power vs weight, or towing power) as both are capable of towing my caravan easily. Neither has as good economy as the Volvo before those with a small 1.4 litre lean burn Mitsubishi engine which could average over 45 mpg on the same run, but then that couldn't tow my caravan (to be honest it sometimes struggled to pull itself).

sometimes that bigger engine can deliver the same power with less effort and thus gets the better economy.

(well that's how I see it anyway)

Grendel

[LondonRascal]

Thanks for that Grendel, I guess too especially in a boat where you may be running the engine for longer at higher revs something that is flat out 1,100RPM less than the other engine might last longer too.

[Oddfellow]

Goodness.

 

If we're talking about boats, you need to also consider the reduction ratio of the gearbox, the design of the hull and the size and pitch of the propellor and the environment that the boat will be used in.

 

There are plenty of 40ft boats on the broads that are powered by a 35hp BMC 1.5 engine - the very same that we have in our 27ft boats!

[LondonRascal]

Of course there are a number of variables - I just found it curious that two engines producing the same (roughly) Horse Power varied so much in the amount of revs required (by 1,100) to produce their peak Horse Power.

 

Some engines of the same design will be tuned to give high outputs e.g. one base engine might produce 30HP but could produce 40HP and the next model up begins at 40HP but might be tuned to produce 60HP.

 

I guess in such circumstances, if you were looking for a 40HP engine it might be worth paying more for one that was 'designed' to produce this, and not a smaller engine' tuned' to be able to produce that output.

 

It all seems a rather mysterious, but none the less interesting world to me.

[Guest]

Yes, the addition of a turbo on a diesel will give a big increase in shaft horsepower, even with a lower cubic capacity and less RPM.

 

The turbo forces air into the combustion chamber under pressure, rather than just sucking it in by vacuum from the pistons themselves.

 

The resulting "bang" when the mixture is ignited therefore has a much greater force, comparable with a much larger cylinder capacity in a normally aspirated engine.

 

...and if we were then to theorise over boat speeds from various horsepower engines, it does indeed get much more complicated, as Freedom said.

 

In fact, if we stay with displacement speeds, a longer, heavier boat can often go faster than a shorter light boat with the same engine size, because the drag of the hull is less with a longer waterline length.

[BrundallNavy]

Robin,

As you will have discovered on the many hire boats you have had, a newer engine does not necessarily mean a quieter boat or more economical to run. Some of the older engines such as BMC or Perkins although noisier tend to be a better choice.

Doug.

[LondonRascal]

The hire boats I have had with the worse fuel consumption have always had Hydraulic drives rather than a traditional gearbox and shaft to the propeller.  Those with more modern Nanni engines may not necessarily be quitter but have been smoother and the power delivery more precise and a faster throttle response with less lag. 

 

I like that.  Let's pretend I had a boat and it was 40ft long and had a BMC 1500 in it and it ran well and would cruise along all day sipping fuel and using hardly any oil - I'd still wish for a new engine - even if it used a bit more fuel, I know it would rarely need oil topping up and the coolant would sit there happily not needing topping up every day or two.  I would not think twice about the low sulphur fuel I was putting in, and if anything was to go wrong it would be new off the shelf parts not reconditioned, scrounged or cloned copies of originals.

 

Having been on many boats and seen how things have been done, then looking at boats that have been for sale and seeing how those have been fitted or engineered I am constantly thinking 'why was that done like that' or 'you could try doing this'.

 

As far as I can see there are three types of owners: Those who would like a new engine but can't afford it, those who don't think twice about it as long as it starts and goes that's what is important and those who have a real love of a certain type or brand of engine and would keep it going for decades come what may.

 

I'm a forth type who obsesses over manufactures websites and engine datasheets and day dreams about what I would do if I could do it - right down to the type of lead lined sound insulation and  having forced air ventilation to the engine compartment to help keep the Alternators cooler and supply the greedy engine with air to breathe and have the hot stale air extracted.

 

But through this thinking comes questions , such as the above as I always thought the larger the engine the less fuel efficient it was  and I think that is true - sometimes but probably not all of the time

[mbird]

Ignoring gearboxes, props, drive types etc and just thinking in very basic terms about the actual engine, in one minute, the volume of air displaced would be;

 

2500 x 4.085L = 10,212.5 litres

3500 x 3.168L = 11,088 litres

 

This isn't actually a measure of the fuel used, as it depends on how the pistons are arranged on the crank (i.e. how many pistons are sucking in fuel per rev as some will be on the exhaust stroke) and how many cylinders there are. Also, it obviously makes no allowance for fueling (common rail or old fashioned injection pump) and aspriation (super charger, turbo, normally aspirated), but does illustrate that the larger engine is shifting less gas. Or at least, I think it does!

[BroadScot]

Who needs to go to college or University when you can do the NBN Open Uni course on Maths and Physics here!

 

 

Iain.

[Guest]

Ignoring gearboxes, props, drive types etc and just thinking in very basic terms about the actual engine, in one minute, the volume of air displaced would be;

 

2500 x 4.085L = 10,212.5 litres

3500 x 3.168L = 11,088 litres

 

This isn't actually a measure of the fuel used, as it depends on how the pistons are arranged on the crank (i.e. how many pistons are sucking in fuel per rev as some will be on the exhaust stroke) and how many cylinders there are. Also, it obviously makes no allowance for fueling (common rail or old fashioned injection pump) and aspriation (super charger, turbo, normally aspirated), but does illustrate that the larger engine is shifting less gas. Or at least, I think it does!

 

Although the relative proportions are the same (so your point is still very valid), it's actually half that volume, 5106.25 and 5544 litres.

 

Four stroke engines take air in on every alternate  revolution.

[mbird]

Although the relative proportions are the same (so your point is still very valid), it's actually half that volume, 5106.25 and 5544 litres.

 

Four stroke engines take air in on every alternate  revolution.

 

That's certainly true for 4 cylinder engines, but is that still true if the engines are 6 cylinder, with the crank aranged at 120 degree intervals rather than 180 degree in the case of 4 cylinder lumps, so there would be 3 power strokes per rev rather than 2?

 

No, forget that, since even if arranged in that configuration, there would still be half the pistons on an intake stroke per rev I guess!

[Guest]

That's certainly true for 4 cylinder engines, but is that still true if the engines are 6 cylinder, with the crank aranged at 120 degree intervals rather than 180 degree in the case of 4 cylinder lumps, so there would be 3 power strokes per rev rather than 2?

 

You've got me nervous now, but I think it's still half the revs.

 

No matter how many cylinders there are, each one requires two revolutions of the crankshaft to complete a full intake/compress/ignite/exhaust cycle.

 

So a full charge for all of the cylinders would take two revolutions, even though they're starting and finishing radial positions are overlapping.

[LondonRascal]

This has got a bit, how shall I say ‘over numeric’ for me and my mind is boggled.

 

Having look around the Web, it seems this has been raised with reference to car engines and there really is no overall answer – there are simply too many factors to have an overall answer that fits all.

 

What is likely though if you have a larger engine that has to rev at a lower RPM to achieve the same horsepower, it will be a quieter experience whether you burn more fuel or not int he cockpit.

[Guest]

......What is likely though if you have a larger engine that has to rev at a lower RPM to achieve the same horsepower, it will be a quieter experience whether you burn more fuel or not int he cockpit.

 

Maybe not Robin...

 

I believe modern small Jap diesels like Nannis and Betas are often  smoother and quieter than old BMC and Perkins diesels, despite revving higher to achieve the same bhp.

 

As you say, it's very complex, with so many evolving design factors.

 

Advances like indirect injection can greatly reduce the traditional "clatter".

[LondonRascal]

Ok yes, good point...Damn.

 

.

[Guest]

A very interesting topic though Robin.

 

I took this photo last year at Stoke Bruerne of an old narrowboat diesel.

 

I was listening to it running for so long that my wife had to drag me away.

 

I'd been hypnotised by the "thump thump thump...." 

[ranworthbreeze]

Hello Strowager,

 

Those Russell Newbery engines are something to behold and have been in production since 1909. I can understand you not wanting to stop listening to the engine. The Bolinder semi diesel engine is another classic narrow boat engine, that is fitted in a lot of the old working boats.

 

Regards

Alan

[Guest]

I had to be dragged away from the motorbikes there too.... 

[unclemike]

ooh a proper bikecomplete with fishtail

[riyadhcrew]

Strowager,

 

My avatar shows an Aprilia Mille R Factory which I love to ride and used to race (amateur). The attached is a bike I found in a showroom here in Riyadh. It is a 2009 and has zero miles on it. In the process of changing all the fluids and the tyres. It's an American Yamaha Raider with 1854cc.

[Guest]

Wow, it really is from a different millennium Eric !

[riyadhcrew]

Still got just 2 wheels and you can still fall off. lol.

[stumpy]

A mate of mine had a Panther 600 single with a single seat Watsonian sidecar (before the compulsory helmet laws) now that was aural Viagra lol

[BroadScot]

Hi Eric,

 

Be careful, Griff will be over and nicking it.

 

 

Iain