Too many weekly miles for a petrol engine?

Turtle-TM

I currently do about 370-450km a week, all 80km/hr n roads and motorway driving, and occasional city driving. My 3.0tdi gets me about 33-36mpg, I put €50 a week in and it just about does, my question is, if I change to a 1.8t petrol (which is actually a 1998cc "biopower" Saab engine) will I see the same mpg or will I be pouring money into it every week? I've never done any major miles in a petrol car so really have nothing to compare to.


Thanks in advance!

corkgsxr

I would think you could get low to mid 30's in and petrol handy enough

wiz569

What about a smaller engine diesel as your driving style/habits lean themselves to diesel rather then petrol

Turtle-TM

If the car I'm looking at was diesel it would be perfect, at least I'd know for sure that I'd get the same mpg. I know that with the decreased bhp ill be taking it more easy than I do in my current car, I just can't resist putting the foot down to over take someone and getting that whoosh from the turbo as I fly past everything! But the low pressure turbo in the Saab gives more spread out power rather than between 2-3k rpm so I wouldn't be putting the foot down to fly awayn

It's just a hard one to know, for me anyway, some people are saying I'm mad to get a petrol engine

unkel

You have a heavy foot! That said, you should still get 30MPG in a Saab 9-3 2l petrol turbo...

Turtle-TM

I'm fairness when I'm on the motorway it's at 2k rpm @120km/h on cruise control, it's only when I feel the need to give the fast pedal a dap and feel the speed! But I can certainly live with out doing that. That said I'd be very unhappy in a 1.4!

I guess I just won't really know till I start driving it and start putting money in the tank! I'm just wary as this is going to be my car for the next 5 years and I don't want to be stuck with a noose around my neck!

derry

Heavy foot and petrol are not good mix for best MPG

Petrol prices will probably in time become more than Diesel again

Long distance with steady state motor way driving Diesel is your only man they sip fuel

Short journeys in heavy to medium suburban traffic often petrol is your only man

If you can handle the bore dom factor the Peugeot Partner delivery Van with diesel 1200cc will easy do 70MPG often it will do 85MPG., Acceleration is crap and top speed is crap but your smiling all the way to the bank if you do motor way driving .

If you train the right foot and keeps the speeds steady you might get the 30MPG on 1800cc but if your like my friend any way hard right foot he gets less than ~25MPG with his 1800cc

Petrol Hybrids reputedly will reduce this heavy right foot problem but they are mad money and for that extra premium price money how much fuel could you have bought

Keep us posted

Derry

corkgsxr

Bigger petrols can be quite decent long distance. Smaller ones cant take the revs and suffer but bigger aren't under pressure and imo 40mpg is quite achievable.

I find small diesels you need to stick to 90kph to get near the figures they quote.

Drive them at 120-130 and youd only get mid 30's

R.O.R

I had a 9-3 1.8t Bio-Power for a couple of days when they were launched in 07. Someone else had been driving it around town for a couple of days before I got my hands on it, so the trip was showing somewhere north of 13 l/100km.

Reset it while doing a fairly steady 100km/h on the M1 thinking it will come back down to circa 7 L/100km (around what I'd expect for a 2 litre Petrol) and it did come down, but only to the high 8's.

With a light foot and the type of driving you decribe OP, you should be able to see similar MPG to your 3.0Tdi from a lot of 2.0i cars, but not the Saab.

For some reason, I've found the fuel consumption from Bio fuel / Flexi Fuel Petrol's to be much worse than that from the same engine designed to be run only on Petrol - the S40 1.8i FF was similar.

OP how about converting the Saab to LPG ?

The Prius MK III on LPG would be the cheapest car to run bar full electric and the MK III Prius has a decent bit of poke in power mode too.

Just a thought.

What is the car you got now ? you might miss all that torque ?

I can get 30 mpg in a 2.0L 2000 CRV, not bad it's got a decent amount of torque which helps.

Matt Simis

corkgsxr wrote: »

Bigger petrols can be quite decent long distance. Smaller ones cant take the revs and suffer but bigger aren't under pressure and imo 40mpg is quite achievable.

I find small diesels you need to stick to 90kph to get near the figures they quote.

Drive them at 120-130 and youd only get mid 30's

Yeah but if he can only get:

3.0tdi gets me about 33-36mpg

out of a 3.0TDI, he hasnt a hope of getting close to that in any normally sized petrol.. or his TDI is not working correctly. Big diesels excel at higher speeds vs small diesels, so I dont know whats going on with the numbers being quoted.

derry

[quote=3.0tdi gets me about 33-36mpg }

Matt Simis wrote: »

out of a 3.0TDI, he hasnt a hope of getting close to that in any normally sized petrol.. or his TDI is not working correctly. Big diesels excel at higher speeds vs small diesels, so I dont know whats going on with the numbers being quoted.

The issues are to do with the way petrol works versus Diesel . A petrol requires lots of RPM to give its BHP .A diesel will return the BHP at often a lowwer RPM. Rpm on any engine reuires the engine to suck in more air .As sucking in air into engine is like sucking in air through a straw there is lots of friction losses .A typical engine will lose 40% of the power from the fuel used just to suck air into the engine and eject it and this is power loss increases exponentially due to aerodynamic factors .So a petrol engine of ~2000 cc which Requires a ~3000 RPM to drive the car at ~70MPH versus a ~3000cc diesel that has RPM of ~1500 will mean the petrol car will lose a lot more than double the power to sucking air in in fact it might lose closer to 4 times the power .

There is also the fact that petrol fuel has less energy in it than diesel some 10% less .

Then there is the fact that Petrol engines operate at compression ratio of 9.5 to 1 compared to diesel engines which often are more than 16:1 .The higher the compression the more efficient is the conversion of the power from the fuels used .

The reason some larger petrol cars can return respectable MPG at cruising speeds is the reality that no matter it is diesel engine or petrol engine two cars doing the same speed for example say ~70MPH will use the same amount of BHP.The amount of BHP needed to drive a medium to large car at ~70 MPH is guite small often less than ~50BHP but if its uphill then you often need a lot more like 100BHP and steep downhill as low ~20 BHP . Over taking and accelerating will require extra BHP and putting the foot down will bring more fuel into engine and make more BHP. As fuel is what gives you BHP the two engine will burn roughly the same amount of fuel to generate the same BHP in steady state flat road driving some ~50BHP (provided we are talking injection engines as aspirated petrol will often use extra fuel ).
However any changes in speeds from heavy right foot or traffic flow interruptions will mean very quickly the larger petrol will fall down as they eat fuel when accelerating but produce lots of BHP where diesel doesn't tend to be so severely effected with changing speed modes as they produce a lot less extra BHP when you put the foot down and so tend to use less fuel .

Much of the reason the diesel engine is better with fuel is the fact they produce often half the BHP of the petrol engine but they do supply the more correct amount of BHP needed

If you got a aspirated 1000cc petrol engine to replace 2000cc non turbo diesel engine both produce about ~75 BHP and lets say we put this into two cars larger size like typical 2000 liter ford Cortina you would find some issues .In steady state driving at 45MPH the petrol would return easy 40 MPG and the diesel the same . They would both produce about ~20 BHP the power needed to push the car along at the slow speed of 45 MPH . However on the hills or overtaking or heavy demands like high speed the smaller 1000cc would have to gear down and suck in lots more air to run at HIGH RPM to give the ~75 BHP where the diesel will produce that at lower RPM .The result is the petrol would return a lot less MPG probably less than ~30MPG due to high RPM losses and the diesel might return closer to ~35 MPG

If you replaced the petrol engine with lets say 3000cc engine then the big petrol would turn over slowly with RPM of probably less than ~2000 RPM to go at 70MPH . Then in steady state driving to make the ~50BHP needed the fuel figures might be OK probably north of ~30MPG .However any deviation from steady state will greatly increase the fuel consumption and any type of heavy foot could bring the figures down to less than ~25 MPG


The use of much cheaper LPG fuels could definitely make a Petrol car cheaper to run than even the diesels (if you do serious mileage more than 15,000 miles every year ) (bear in mind that LPG gives less MPG than petrol or diesel fuels its only its very cheap that makes it work )


Because the car are effected by air resistance and that Quadruples with the doubling in speed the smaller cars suffer more from going at higher speeds like 70MPH and have to use high RPM where engine losses are higher for the smaller engines . This impacts the mpg both in petrol and diesel. Ball park the shape of car is not interesting below speed of 30MPH .Most cars will return best MPG at 55MPH as air resistance is still not big an issue. However the air resistance at ~70MPH will impact the MPG a lot more depending the shape of car the more square shaped the back of car is the worse this will be ,as it the way the air exits the car the makes the most drag . A square shaped rear end will create a large vacum effect sucking against forward movement so more BHP is diverted to combat this vacuum effect . The idea sharp nose or blunt nose is that important is not true until the speeds exceed ~100MPH as air moves out of the way easily enough .Its the shape from back of car that is more important at 70MPH A very Pointed rear end will help more than square shape like van .If most modern cars drove backwards at 70MPH they would save ~20% of the fuel they use as then the pointed end would be at the back of the car .

here are some number I have got from my cars
All done on motor way or cabbage way steady state over several hundred kilometers to even out figures

1998 1000cc Susiki swift (aspirated petrol)

40MPH ~= 65 MPG all 5th gear
50MPH ~= 50 MPG
70MPH ~= 35 MPG

1999 1400cc fiat Bravo (aspirated petrol)

40MPH ~= 45 MPG all 5th gear
50MPH ~= 35 MPG
70MPH ~= 28 MPG

1973 2000cc Toyota Carina aspirated diesel Automatic(gear/box ~10% better)

40MPH ~= 35 MPG Automatic gears
50MPH ~= 35 MPG
70MPH ~= 33 MPG

Note the similarity between small 1000cc and the the larger 2000cc diesel at 70MPH

Note the very good MPG for the petrol's steady state at 40MPH and how quickly the petrol use increases with speeds

Bigger diesels at ~70 MPH in steady state driving is your only man especially if your heavy right foot guy

I dont have the numbers from modern Petrol larger engines steady state but the hyper miler groups expose light right foot is essential to keep petrol's MPG good any type of heavy right foot and forget it


I suspect the SAAB using the Bio mass engine has other special issues .The engine meant to burn the bio fuels has to supply 30% more fuel to engine when using bio fuels as bio fuels supply 30% less power density .It probably means the engine has to assume the fuel is bio fuel and inject in 30% extra fuel until the computer can figure out what amount of fuel is Petrol and what is bio fuel . Once the computer figures that out in steady state it will return to seeing the fuel is normal ROI petrol, 5% bio fuel and 95% petrol .However there is probably no memory in the computer so every time the speed changes the computer goes back to trial and error to look the fuel and throws in 30% extra fuel until it sees the new fuel numbers . Ok it its only seconds every time but the more the speed changes the more fuel gets wasted in these seconds is my guess .
Couple in that any engine that is made to run on Petrol only will be tuned to perfection to run on petrol.Any engine made to run on bio fuel will be tuned for perfection to run on bio fuels and that will mean 30% more fuel injected more advanced ignition setting larger spark plugs .
Any engine that will burn the two fuel will have to compromise and not be so efficient with petrol or not so efficient with bio fuels and probably most efficient in the mid point where its 50% bio fuel and 50% petrol .( Bio fuel is no longer available in ROI ) Its probably too expensive to sort out the SAAB bio fuel engine problem just for ROI so you just got to live with less MPG is my guess

Derry

corkgsxr

Your wrong in so many ways its not even funny. Its not pumping losses that affect efficiency its friction of internal moving parts.

Theres no different type of bhp. A petrol and diesel develop power in different ways.

I find efficiency comes down to revs as long as your not too low.

A diesel at 3k revs isnt great at all on diesel.

Acceleration in petrol or diesel both use similer fuel.

People have mad ideas about engines

Turtle-TM

The best I get in my 3.0tdi is 7.4l/100km which is around about 38mpg. If I could even get 34-36 from a petrol I'd be happy.

Apparently the Saab senses if there's bio ethanol in the tank and adjusts accordingly, so it's not constantly pumping extra fuel into the engine.

I'm happy keeping it around or under 120km/h on motorways, I generally do 130 in my current car. The loss of torque I'm not worried about, I know you can't dab the fast pedal and get instant torque on a 2.0 petrol.

From what I can tell its swings and roundabouts, some are saying it'll eat fuel on a motorway, some saying there's not much difference. At 10cent a litre at the pumps it's not a huge difference there.

derry

Try this hyper miler site out see if they can really get 35MPG at 120KPH with 2 liter engines might even find a SAAB there

http://www.hypermiler.co.uk/forum/

Derry

derry

corkgsxr wrote: »

Your wrong in so many ways its not even funny. Its not pumping losses that affect efficiency its friction of internal moving parts.

Theres no different type of bhp. A petrol and diesel develop power in different ways.

I find efficiency comes down to revs as long as your not too low.

A diesel at 3k revs isnt great at all on diesel.

Acceleration in petrol or diesel both use similer fuel.

People have mad ideas about engines

If we discussing only the engine it would be simple such as static generator making electricity
However its the interplay between the human operator and the engine and the car that makes the story get very complex

The basics of engines are they are 25% to 30% efficient with fuel
However there are frictions from the car
Rolling resistance remains relativity static no matter what speed
Wind resistance increases by the quadruple with doubling of speed
Disk brake drags on the disk can be as much as 5% of resistance to forward travel
When all Resistance is combined the global return on efficiency of car is on highways steady speed about ~12% in mixed traffic ~5% and in traffic jams a lot less than 5%
Because the diesel engine cars are heavier in town traffic they often return disappointing MPG in urban traffic often better than petrol but nothing worth talking about
In steady state motor way driving the diesel engines will often return better results

Inside the engine the majority of the losses are frictions like crank shaft on bearing etc .Many of these frictions are linear they don't increase much with speeds similar to car rolling resistance .Some frictions will increase with changes in engine speeds but often not in a exponential curve eg the doubling the speed will make the friction double .
However in pumping air or gasses the frictions are ball park 40% of engine frictions and can often be exponential in nature as they follow the rules of aerodynamics .The rules of aerodynamics within a complex engine are fairly complex within tubes with skin friction compression frictions and expansion frictions and speed drag frictions .Basic aerodynamic rules is doubling the speeds will result in quadrupling of the friction resistances.Inside the engine much of the pumping friction will tend to quadruple when the speed of pumping is doubled .

when a static diesel and petrol engine electric generator supplying the same kilowatt load are suddenly required to to supply the same extra load they will accelerate the fuel supply to supply extra fuel needed to both engines at the roughly the same rate

In the car world there is difference because the nature of the right foot

The two cars petrol probably 2000cc ~150BHP and the turbo Diesel probably 2000 ~100BHP both going at steady speed of 70MPH will be producing about ~50 BHP
When the petrol head floors the throttle pedal the result is the engine often goes to full power and that full power exceeds the diesel engine full power by often larger factor .As BHP power comes from fuel burned more fuel ingested into engine will return more BHP .Often the petrol head doesn't ease of the full throttle after over taking and so quickly start to result in more fuel being used as he isn't matching the engine demands so well to the road conditions

If humans could act like robot controls like we see in static electric generator production then fuel figures would often be similar between diesel and petrol cars .
However as many hypermiling forums will tell you the human right foot throws out huge variables in MPG
In the reply I try to keep the complexity off the problems to human car motor interactions levels simplex so as not to write a history of thesis on the subject and confuse people too much

So for those reasons i will say
Pumping forces losses can be reduced a lot with lower RPM ignoring other engine friction losses as many of those are linear losses and there inst much that can be done to reduce those losses .

In the real world the right foot causes the large differences in fuel consumption between diesel engines and petrol engines which has very little to do with the fact that both engines are very similar in fuel demands for the work loads but often humans don't seem to be able to match these demands properly and the more testosterone often the more the MPG falls with petrol engine cars

But if i have made some serious error please enlighten me I for sure don't know everything and am always interested to learn a new tricks


The best expression sums it all up "the weakest link in the car is the Nut behind the wheel "
Derry

corkgsxr

Pumping losses is a tiny factor. It that was 40% of friction or 40% of efficiency losses car manufacturers could easily double the size of valves and inlet tracts. Its not the problem. Efficiency losses are through heat and friction of moving mechanical parts.

As speed increases wind resistance plays a big part but smooth curves aren't the most efficient way to push through the air. Recent developments have discovered angles that deflect air away from the cars body is better that a smooth blob of a car.

Reason lower rpm uses less fuel is that its pumping in less fuel not pumping losses are lower.

It just goes to show alot of hypermiling advice is based on guesses and imo not facts. And hypermiling a diesel will kill it. Your lowering running heat so you block dpfs egrs intake manafolds etc. Alot of the techniques will raise mpg in the short term but will cost more in mechanical parts.

derry

corkgsxr wrote: »

Pumping losses is a tiny factor. It that was 40% of friction or 40% of efficiency losses car manufacturers could easily double the size of valves and inlet tracts. Its not the problem. Efficiency losses are through heat and friction of moving mechanical parts.

As speed increases wind resistance plays a big part but smooth curves aren't the most efficient way to push through the air. Recent developments have discovered angles that deflect air away from the cars body is better that a smooth blob of a car.

Reason lower rpm uses less fuel is that its pumping in less fuel not pumping losses are lower.

It just goes to show alot of hypermiling advice is based on guesses and imo not facts. And hypermiling a diesel will kill it. Your lowering running heat so you block dpfs egrs intake manafolds etc. Alot of the techniques will raise mpg in the short term but will cost more in mechanical parts.

Thanks for the informative reply .
I shall have to go back to source what supplied that claim that ~40% of losses in the engine came from turbulence or pumping losses. Without measuring equipment it isn't easy to figure out what losses they are .

Not related to car but in fact a flat faced cylinder shape at hypersonic speeds will have less drag than a pointed nose as thaat will deflect the air around craft and remove skin friction however there will be no real way to control as any fins would not grip the air or fluid medium.

I haven't heard of any benefit to be gained at slower speeds where car operates to deflect air forces away from car will look into that.

I agree there does seem to be some evidence that diesels don't like to operate at low RPM regimes under loads so will need to look into that as no point to get false saving and reduce life of engines

Derry