Larger displacement for better mpgs?

[kachok25]

Here is an intresting topic, we were all tought that smaller displacement engines made less power but got better fuel effecency, and larger engines would always burn more fuel, back in the days when a Holly 750cfm was high tech that was certainly the case, but now we are starting to see the opposite in many cases. I work at a chevy dealership and I noticed several examples of this. The Colorado with the Z71 offroad package gets better effecency with the inline 5 than the base model with the four banger (crew cabs), the Tahoe gets better effecency with the larger 5.3L than the 4.8, and the full size Impala with its 3.5L V6 only gets one mpg less than the tiny cobalt with its wimpy I4. The Z28 for its weight has to be one of the most effecent cars of all time, even before displacement on demand the nearly two ton cars was getting 28mpg with proformance gearing and tires!! And lets remamber that it was a front engine rear wheel drive so it had alot more powertrain loss than an FF car. I may not be an engineer but it seems to me that higher displacement engines that run much lower RPMs have much more potential to be effecent. OK here is my questions, how much energy does an engine loose in friction from the swept area, I have heard that if speed doubles the friction in the swept area is increased four times is that true?
Question #2 in the avrage engine there is supposed to be around a 63% thermal loss, say we cut the surface to volume ratio in half, aproxamatly how much more effecent would it be all other factors being equal?
Could someone build a very lage bore engine that could run highway speeds with 500-800 rpms and get better ecconomy than a Toyota 1.3L inline 4, or is there a law of deminishing returns that I am overlooking?

[kachok25]

Lets assume that I had the resources (aka money) to build a project engine lets say I build an inline or horazontaly opposed four cylinder with an absolutly insane 7 inch bore and 6 inch stroke, we would have over 800 cu in displacement, and about half the surface to volume ratio as a 5.7L V8, mathamaticly speaking we could put it in a camaro and run highway speeds at or below 700 rpm, in addition to the crazy low opporation speed we would have an exisessve amout of power just a downshift away, I know that an engine with pistons that size and a stroke that deep probably would come unglued past 3500-4000 RPM, but the upside is that you would probably never need any more revs than that since hp is just a multiple of torque (torque=hp at around 5200 rpm) even if could only make 900 ft-lbs (direct injection with around a 12.5:1 CR I would say 900 is a very safe bet) and we maxed out at only 3500rpm we would still be making over 600HP! I don't think weight would be a problem with this engine, we have about the same surface area as a small block v8 and the LS2 fully dressed weighs less than 400 lbs, so it is not out of the realm of possability to make our project engine a comprable weight. This engine could not be made street legal because of emissions laws, big bore/long stroke engines tend to have a problem with unburnt hydrocarbons, but with the advent of gasoline direct injection, and multi pulse injectors, not to mention the use of E85 a street legal engine of this type might be possable. Does anyone out there in internet land see any problems with a project like this?

[GreyGoose006]

well 4 cylinders at inch bore and 6 inch stroke gives you 15.1 liters.
i think the sheer size of that engine might ruin any hopes of good gas mileage. just think, you need to fill those enormous cylinders with 15 liters of air and fuel.

you would have a huge problem with the flame spreading.
i dont know the technical term, but with larger bores, the amount of time it takes for the flame to spread and ignite the fuel actually becomes a variable.
you would have to use multiple spark plugs and have to advance the timing so that the flame was not still spreading as the cylinder reached the bottom of travel.

essentially you are talking about building a boat motor into a car.
personally, i would use a 3-4 cylinder diesel with a massive bore and short stroke (for the size of the bore)

a 5 inch bore with 4 inch stroke and 3 cylinders gives you 4 liters.
put that into an any early 80's japanese sports car, and you have yourself a mileage queen. (hehe)

your unburnt hydrocarbons could be taken care of by using multiple spark plugs (on a gas) or high compression (on a diesel)

[2.2 Straight six]

If you want better fuel economy, buy a Volvo 2-litre diesel. 48mpg combined.

Case closed.

[Steel]

or to make 2.2's statement broader, if you want fuel efficiency, you cut gasoline out of the equasion, and go for lean-burn compression ignition fuels aka diesel.

[kachok25]

Originally Posted by GreyGoose006

well 4 cylinders at inch bore and 6 inch stroke gives you 15.1 liters.
i think the sheer size of that engine might ruin any hopes of good gas mileage. just think, you need to fill those enormous cylinders with 15 liters of air and fuel.

you would have a huge problem with the flame spreading.
i dont know the technical term, but with larger bores, the amount of time it takes for the flame to spread and ignite the fuel actually becomes a variable.
you would have to use multiple spark plugs and have to advance the timing so that the flame was not still spreading as the cylinder reached the bottom of travel.

I think the technical term you are looking for is flame front velocity, I don't think that will be nearly the problem you think.
Reason #1 direct injection sprays directly onto the sparkplug(s) so the flamefront would not have to reach into the far edges of the cylinder to catch up with all the fuel, plus with a multi-pulse injector, and a multi-spark ignition I don't think buriing the fuel will be much of a problem.
Reason #2 Since this thing is in theory swinging massive 7" pistons it's opperating speed would be much slower than a 1.6L honda I4, the flame would have several times the amount of time before we hit BDC, remember I am talking about 500-700 RPM crusing speed.
Reason #3 Last I heard a higher CR helped the air/fuel mixture burn faster, so by using a direct injection system which allows for a very high CR (and hotter spark) we should be able to keep the fuel nearly completly burnt up to our 3500-4000RPM redline, afterall if the flame front can travel 4" at 7000RPM why not 7" at 3500??
As far as the huge amounts of power it takes to move 15.1L of air per rotation, think about it this way my 03 Rodeo has a 3.2L V6 that has to turn 2600 RPM on the highway just to maintain 70mph (3.2L x2600RPM/2 = 4160L of air per min assuming 100%VE. My concept engine would be 15.1L x500RPM/2=3775L per min assuming 100%VE. Now which one sounds like more of a gas guzzler? The difference is that the larger displacement engine will loose less energy through the cylinder walls, loose less energy through swept area friction (rings rubbing the cylinder walls), loose less energy through valvetrain friction due to softer springs in the cam (remember around 4k RPM would be this things max anyway) ,and loose less energy through blowby (less ring to leak around per cubic in), so in thory the project engine would be noticeably more fuel effecent and have aprox three times the HP and over four times the torque as my modern DOHC V6!

[kachok25]

Originally Posted by Steel

or to make 2.2's statement broader, if you want fuel efficiency, you cut gasoline out of the equasion, and go for lean-burn compression ignition fuels aka diesel.

BTW direct injection gas engines can lean-burn by injecting just a small amount of fuel right around the sparkplug Mitsu engineers ran a gas engine with as low as a 30:1 air to fuel ratio using direct injection, that is well under half the recomended 12 or 13 to one ratio.

[Moppie]

Four words: Power to Weight Ratio.

A small engine in a heavy car has to work harder to make it move, and therefore uses more fuel.
A big engine in a small car doesn't have to work as hard, and so uses less fuel to achieve the same performance.
I used to have a 700kg car, with a 100ish hp Toyota twin cam in it. If driven gently it used to get crazy mpg, 40+ easily. But if I opened it up, I could drain the fuel tank just driving it home from work.
Basically a 20L tank could last me 7 days, or half a day depending on how I drove it.

[KiwiBacon]

The smaller engine will always give better fuel economy if run in the same way.

But if run with an auto which kicks down sooner (effectively lower gearing) and a computer which richens the mixture a lot when full throttle is used, then the comparison becomes distorted.


As far as efficiency, a larger engine can be more efficient than a small one for the reasons you mentioned (volume to surface ratio). But a large engine requires a large load to run efficiently (otherwise the standby losses are too big a proportion).
Running a large engine with a large load puts economy right out the window (even though efficiency has increased).

For example.
Isuzu have a 2.8 litre diesel engine which has a lowest BSFC (brake specific fuel consumption) of around 240 g/kwH.
Isuze also have a 4.3 litre diesel engine which has a lowest BSFC of around 215 g/kwH.

Of course diesel has a wide and flat efficiency curve, petrols have a small island of best efficiency whch is very easy to miss.

[kachok25]

Originally Posted by KiwiBacon

The smaller engine will always give better fuel economy if run in the same way.

But if run with an auto which kicks down sooner (effectively lower gearing) and a computer which richens the mixture a lot when full throttle is used, then the comparison becomes distorted.


As far as efficiency, a larger engine can be more efficient than a small one for the reasons you mentioned (volume to surface ratio). But a large engine requires a large load to run efficiently (otherwise the standby losses are too big a proportion).
Running a large engine with a large load puts economy right out the window (even though efficiency has increased).

A few things you are not taking into consideration standby losses are reduced with a direct injection (remember the 30:1 AF ratio) Also remamber we are in the age of hybrids once the battery is charged who is to say it wont have an auto shut off, and just run it on the battery for a while, but even with regular port injection and no hybrid system I would put it aginst any proformance minded 4 cylinder anyday, for effecency or proformance I would bet the larger engine would stomp a mudhole in it in every event exept for standby fuel consumption, and you know what I could live with that. I don't know about you but I don't spend much time at a dead stop. During my hour drive to work everyday I probably do not spend any more than three munites at a stop.

[Steel]

30:1 is nothing. Diesels regularly idle at 80:1 or higher, and get down to 15:1 when under full load. It's easy once you only have one variable to change, the amount of fuel, as opposed to a gasoline engine, fuel AND air.

[kachok25]

Originally Posted by Steel

30:1 is nothing. Diesels regularly idle at 80:1 or higher, and get down to 15:1 when under full load. It's easy once you only have one variable to change, the amount of fuel, as opposed to a gasoline engine, fuel AND air.

It would be so much easier to just make it a diesel and get it over with, the harsh reality is that most people hate diesels, I don't I think they are great, compression ignition takes out so many dificult challenges: hot spots, swirl heads, squash area, ignition timing, load vs comprssion compromise.... it just makes too much good sence, but if I built the perfect diesel 85% of the American population would not give it a second look I dont care it it got 3,000,000,000,000 mpg and the exaust smelled like flowers. Untill the American public has a change of heart they are going to buy gas engines.

[Steel]

Originally Posted by kachok25

It would be so much easier to just make it a diesel and get it over with, the harsh reality is that most people hate diesels, Untill the American public has a change of heart they are going to buy gas engines.

Becasue they are stupid. SHeeps that let the EPA and Society of Concerned Scientists (a crock!) make their opinion for them. Never mind that VW/Audi is making a new SUV with a 6 liter V12 TDI diesel making 500 HP and 738 torques! and still gets 20mpg!

[kachok25]

As completly sweet as that is I am trying to beat that for power and fuel effecency, without the use of a turbo. With less than half the friction, and half the heat loss of a conventional V8, I need to figure out how much energy is lost through the cylinder walls in in swept area friction berore I can give a solid estamate on fuel effecency, but I do know that a 20mpgs is not unrealistic, an 07 tahoe with a 5.3L 320hp V8 gets up to 22mpg. SO if anybody has any figures on swept area friction on thermal loss let me know. Another concern that I have is the rev willingness on such a long stroke engine, even with it's insane power if it does not behave like a proformance engine with WOT it will not catch on. Even with a high CR, undersquare bore to stroke ratio, and lightweight forged internals, I am still not sure it will rev like a V8. Using either an in line or horozontaly opposed design it sould not require any counter weights, that will help a little, but the prospect of 7" postons probable weighing better than 2lbs each sounds very imposing, especaly since nobody that I know of has ever tried anything like this that I am aware of. Oh I am sure that someone else in history has built larger pistons but not for combined proformance and fuel ecconomy.

[drunken monkey]

PERformance