Is there a source for optimum engine rpm

[kane66]

some_other_dave.... I really do understand that optimum fuel efficiency is the some of everything going on at the time, thats why I'm transplanting a 1.6 out of a german box into a much more stream line civic hb... but all things being equal. a 1.9l TDI will get 25% better fuel economy than a 1.6 TD at 1800 rpms. Thats what I'm getting at and thats what I'm looking for. If we had a list of ever engine and their respective optimum fuel efficiency, you could literally "pick out" the very best engine for you application, then take care of all the other considerations knowing that your starting with a strong base.

[Sideblinder]

The most fuel efficient engine for your car is the engine which produces, at it's peak, the peak power you'll need. The reasons why:

The average car engine is most efficient (fuel vs power) somewhere med-high up in it's rpm's. This is because of the way we control the engine. We limit the amount of air it can suck in by placing a door in the way (the throttle plate). This applies to gasoline engines only, diesels are different but the general rule at the bottom will still apply. This creates drag. More then enough drag to really hurt the fuel-power efficiency. As we open the throttle, this drag is reduced, and the fuel-power ratio increases until it peaks, then internal inefficiencies in the engine eventually start bringing it back down.

So you see that it will be most efficient at turning gas into power when you have your foot all the way down and the throttle plate completely out of the way . Unfortunately, unless you want to accelerate, this will produce way too much power.

You may find the most efficient engine in the world, but unless it's matched to the car weight/transmission/drag such that it's efficiency points are utilized, somebody who just picked the smallest engine that is sufficient to move the same car will get better mileage.

[kane66]

sideblinder.... now that is some useful information. I knew that gas engines restricted air intake during low load situations. I wonder how much fuel that wastes and at what point you overcome it. I guess that's why those BFSC look so odd. A diesel engine is much different. It doesn't waste fuel until you can't cram enough air in to burn what in there. With diesel you could go with what is considered a horribly undersized engine and still get good fuel efficiency, as long as you can do most of you driving in the optimum Fuel to horsepower rpm. This fact is actually what trips a lot of people up when it comes to diesel, they think that a smaller engine will get better fuel efficiency. which is not the necessarily the case, engine design is the biggest factor determining engine efficiency in a diesel car. I guess any kinda of list showing the relative efficiency of different engines would only work with diesels, as there are to many variables when it comes to gas engines.

[MechEngVT]

kane66,

It seems to me there's something key that you're missing from the relationship between engine design, BSFC, and vehicle fuel economy. The throttle plate in a gasoline engine doesn't "waste fuel" per se, but it reduces the engine's overall efficiency by increasing the power required to pump air through the engine. This restriction becomes less of a factor near wide-open-throttle, or at the last 15% or so of throttle opening before WOT.

BSFC does not correlate with vehicle fuel economy. The 1.9L TDI may have a better peak BSFC than the 1.6L TDI but remember what BSFC stands for: Brake-specific fuel consumption. Units are lbs fuel per horsepower-hour or grams fuel per kilowatt-hour. When you put an engine in a vehicle at steady-state highway cruise on flat ground the vehicle determines the power required (the brake portion of BSFC). The gearing and the cruise speed determine the engine RPM. Looking at the BSFC chart for that particular RPM you must find the load point, which would be roughly the horsepower your vehicle requires to maintain speed divided by the peak horsepower that engine can produce at that particular RPM (% load).

That load percentage would be higher on a less powerful engine like the 1.6L than it would be on the 1.9L. Higher loads usually yield lower BSFC. Depending on what efficiency island that places you on you can not necessarily say that the 1.9L engine would give you better efficiency because it has a lower peak BSFC.

Your statement about comparing diesels to gasoline engines is false. Gasoline engines can be measured in the same manner as diesels relative to BSFC. True, the graphs look different due to the efficiency loss associated with throttling at partial load and the expanded peak efficiencies afforded by diesels' increased compression ratio and turbocharging. Neither design of engine "wastes" fuel, they both convert fuel into rotational energy in different ways at different efficiencies.

I'm somewhat of the opinion that there is far more vehicle fuel efficiency to be gained in properly engineering an engine's application than most realize. Many think that plucking a more efficient engine from another vehicle and dropping it in a less efficient vehicle will improve FE, and it may, but actually *matching* the engine to the application properly will yield much improved results.

[tasdrouille]

I would just like to make a precision on the 1.6 TD vs 1.9 TDI. We're comparing 2 different engine designs here, one's prechamber injection while the other uses direct injection, which is inherently more efficient.

[kane66]

I'm trying to rap my brain around this so I would like a little clarification.

MechEngVT: your saying that if I put a 1.6TD and a 1.9TD in the same car, and geared them to travel at 55 mph at there best specific consumption. Even though the specific consumption for the 1.6TD @ 1800 rpms is 265g/kwh, and the 1.9TDI's specific consumption is 204g/kwh @ 1700 rpms, that the 1.9 might get worst fuel efficiency than the 1.6.

tasdrouille: Thanks for clearing that up for people posting on this thread, I guess I should have said something about that earlier, and actually thats kinda the point I'm trying to get across. Each engine design (even engines in the same class) have inherent efficiency differences, and If you were going to choose a engine for a fuel efficient ICE car, you would want to choose the engine that both fits your needs and is the most effiecent.

[Concrete]

I believe what he is saying is that a tiny engine running in the middle of the power band is more efficient than a large engine idling along at 1700 or 1800 RPM regardless of whether is gas or diesel.

Ideally we install little lawnmower engines in our cars and run them at +60% throttle in cruise (peak BSFC)
but since most of us don't want to die merging into traffic on the freeway we have over capacity
this over capacity is often over 10 to 1 what is needed in cruise

your engine is rarely at peak BSFC
unless you are climbing, pulling or for the few seconds of acceleration up to speed

hope this is helpful

[MechEngVT]

Concrete is right.

You can't really say that a 1.6 TD has a BSFC of 265 g/kW-hr at 1800 rpm. That statement is only true at a specific engine load that will generally be between 75% and 100& load. If this engine will put out enough available power at 1800 rpm so that your cruising load is only 50% throttle, you will be consuming far more than 265 g/kW-hr at cruise. Same goes for the 1.9 TDI.

Given that the 1.9 TDI will put out more power than the 1.6 TD at a given geared engine speed at a given road speed in the *same* vehicle (identical cruise power requirement) the 1.9 TDI will operate at a much smaller percentage of engine load than the 1.6 TD.

Since I can't locate a BSFC chart for the 1.6 TD in an amount of time I'm willing to devote to the search I can't see whether there are plausible scenarios to indicate which would be more fuel efficient in a given application. Generally engines of this size in a compact vehicle application cannot be forced to operate at a high enough load to achieve the exact operating point required to deliver peak BSFC in a cruise scenario. Put the engine in a full size truck however and you just might hit that point.

[roflwaffle]

Originally Posted by MechEngVT

Since I can't locate a BSFC chart for the 1.6 TD in an amount of time I'm willing to devote to the search I can't see whether there are plausible scenarios to indicate which would be more fuel efficient in a given application. Generally engines of this size in a compact vehicle application cannot be forced to operate at a high enough load to achieve the exact operating point required to deliver peak BSFC in a cruise scenario. Put the engine in a full size truck however and you just might hit that point.

VW has enough in the way of gearing and wheel sizes that either can be geared to be at peak BTE at a cruise. Also, the VNT and change to DI allow for a relatively broad, in terms of 300-200g/kWh BSFC map so even w/o the change in gearing a 1.9 TDI would probably be as efficient as a 1.6 TD. That said, gearing options definitely vary by engine type so having a nice range of gearing options in terms of efficiency doesn't always hold true.

[Amiouf]

I have studied gas engines a little. Even if the engine is maintaining its speed on the highway (low torque), it needs a minimal rpm so that an oil film remains between the crankshaft and the bearings. Going with an extra-low gear on the highway would be nice for fuel economy, but each time there is a small hill, the driver would need to downshift. Car manufacturers would probably be able to install a torque indicator requesting the driver to change gear (or an auto transmission would change all the time), but that would be annoying to most people, so we are stuck with approx. 2,800rpm at 60mph, which is somewhat ok for the average driver.