1.5 Potential...

[lyeinyoureye]

Certainly different engines have different baseline engine efficiencies, but I'm not under the impression that with regard to diesels that engine efficiency is necessarily displacement related.  Please correct me if I'm wrong, but because diesels can run infinitely lean and have no throttle, the only downside to increased displacement (given all other factors remaining the same) is that there might be more reciprocating mass or even more weight to carry around.

There is, but it's not the same as gasoline engines because of of what you mentioned, WOT equals minimal pumping losses. If you look at a map of BSFC compared to load, you'll see that as load decreases, diesel efficiency decreases, linearly. For most gasoline engines, as load decreases, efficiency decreases exponentially. So there's a much smaller fuel economy penalty for increasing the displacement of a diesel engine compared to a gasoline engine. But it is present.

[Turbinepowered]

*Sits in a corner, taking notes...*

 Man I love this place.

[Turbinepowered]

lyeinyoureye, I appreciate the info.  It brings up a coupla questions.  I assume that when you talk about load, you are refering to the percentage of max hp/torque that an engine develops is that correct?  When I think of "load" I think "a certain amount of work".  If my definition of load is what you mean, then a big engine puffing up a hill at a specific speed would be under the same "load" as a little engine puffing up the same hill at the same speed.  Whereas if it is referring to the percentage of power an engine is capable of producing than the big engine would be under less load.

Andrew

Usually the latter is what I hear when people are talking about "load" with respect to engines. How "loaded" an engine is usually seems to refer to what percentage of its maximum output it's generating to cope with demands placed on it.

Which, incidentally, is where I was thinking with the "smaller displacement/smaller baseline" bit, kinda. Smaller engine run closer to its maximum output more frequently, rather than a larger engine that dips into the upper ranges of its output less frequently, and therefore is rarely as "loaded" as the smaller engine.

[speedy]

Regarding larger and smaller diesel engines potentially offering the same efficiency at the same load - I offer another data point.  If this were true, there would be no reason to design/build/sell small diesel engines!  Since there are in fact many small diesel engines in service, lyeinyoureye's arguments must have merit.   :wink:

The downside to going with a smaller engine for greater efficiency is, of course, that you have less maximum power available when you need it.  So, like everything else in life, we compromise.

To me, a small engine with... a little extra booOOOOST is a good compromise.  

-Dave

[jtanguay]

well check this out:



I'll let this pic do all the talking... in terms of fuel economy, I would have to say that if you put a big V8 diesel in a small automobile, and found a transmission that kept rpm's near idle at highway speeds, that you would achieve as good or even better mpg than with a 1.5 running at the same speeds.

the fuel economy is lost in the upper rpm ranges.

[jimfoo]

One other point not yet mentioned is that the larger the engine, the more frictional losses you will have, weather it be from more ring/piston contact because of larger bores, larger bearings, more cylinders & cam lobes. These add up. Larger engines will have more rotating mass, using more fuel to accelerate. You won't get a 7.3 liter engine getting 50 mpg.

[lyeinyoureye]

A 1.5L compared to a 1.6l is slightly more efficient, and unless the driver is constantly going near redline/peak power on the freeway, there won't be any drop in efficiency from using the smaller engine. But, the gain in efficiency isn't very large either, since we're talking about a 100cc difference in displacement. I bet having the tires 5-10psi under the sidewall spec would hit efficiency the same, if not more.

Regarding a larger V8 diesel, we could optimize it by having it idle down the freeway, but these are really small cars, so even at idle, the V8 diesel probably wouldn't be able to use enough fuel to compete with the smaller diesel unless the smaller diesel was completely wound out at the same speed. It's just too big, and given the small load required, probably wouldn't have very good efficiency. It'd still get ~30-40mpg, but I don't think it'd ever see 50mpg imo. A compromise would be an engine that's larger, but still small enough to run at high load with the right gearing. Something around 3.0L at ~1000-1500rpm at ~55mph. The TDIclub forum has some interesting posts about swapping in a taller 6th to increase mileage.

Load, in every BSFC map I've seen, is just expressed as torque, or pressure. It's the percentage of torque needed at whatever rpm. The key is rpm, since we're talking about fuel efficiency. Too high, and friction losses build up, too low and we may see other problems like oil pressure or a crappy spot in the BSFC thanks to a turbo (TDI map has this I think). Ideally, we can take map, like this on for the TDI

Or this one for a gasoline engine in one of my cars...

And find a point where the fuel consumption is smallest at some speed. But, we don't want to be upshifting every time we hit an uphill grade, so ~1/2-2/3rds of load at a proportionally lower rpm is a good rule of thumb. Granted, we may upshift more often, but the mileage increases, especially for a gasoline engine, can be huge. For instance, my engine is the dotted lines on the second map. If I can put in a custom OD gear that's half the ratio, my mileage will go from slightly over 30mpg@2000rpm@55mph, to slightly under 50mpg@1000rpm@55mph. A very nice increase. Otoh, the TDI won't see that big of an increase, but there's still something to be gained by taller gearing as well. And if we want to accelerate, we just down shift and take off... Course, most people expect "good" acceleration in all gears, and they wouldn't want to sacrifice "driveability for an extra 5-20mpg, so we don't see really tall OD gears on anything except muscle cars that are trying to avoid the gas guzzler tax (.5:1 sixth gear on a T56). And most economy cars come with CVTs, but it's still nice to know what's possible.And interesting tidbit, is that when making a car more efficient (lower Cd/Crr), we make the engine less efficient assuming the same gearing. This isn't so bad in a diesel, since engine efficiency drops linearly for the most part, but in a gasser, engine efficiency drops exponentially, so it's a definite 2 steps forward 1 step back deal.

[Turbinepowered]

So what you're saying, Lye, is that really I should be looking at that transmission, rather than the engine, to go for super mileage?

So I need to go looking for a BSFC map for the 1.5 engine and do some plotting to find that 2/3rds point you were referring to... Anyone care to offer some help?

[jtanguay]

I would agree... I can't wait to install the PD engine I'm getting.  6 speed tranny... I'm hoping that since its European, that the tranny will run around 2000 rpm @ 80mph or so.  at those rpm's you can just imagine the fuel economy driving 70 mph. (i dont need to drive 80mph... especially with the new OPP stance)

[worf81rab]

What about a 1.9 TD since you're talking about larger displacement?

Also the March issue of Performance VW magazine (its a UK mag you can get at Borders/Barnes and Noble) Has an article on swapping a newer 6 speed into a Mk 2 Golf including a list of all the parts you can source from other dubs

[jimfoo]

Screw the tranny swap, just get 32" tall tires like I have.  :wink:

[lyeinyoureye]

Turbinepowered, don't look into my rants to much. It was more about the difference between gassers and diesel. Unless you have a gasser or your diesel is pretty large in terms of displacement I wouldn't bother with lower gearing. Generally, lower gearing is better, especially when looking at the viscosity of today's fuel compared to the fuel used back when the cars were made (more load and less rpm means the problem with timing advance/viscosity isn't as big, but you can always add oil/parrowax or adjust the pump), but for a small diesel it's not a big deal. For a gasser otoh, it can really help out with efficiency. It will help a little bit, but I don't think many would consider it worth the trade off in power available.

What you should start with is buying some tires with a low rolling resistance coefficient (Crr) and overinflate them ~10psi, or even overinflate your stock tires. Get some kind of sheeting or rubber material and cover up the underbody of the car, and check out this site and this post specifically for more stuff to do. The mods he has listed increased the fuel efficiency by ~30%, but those same mods on a diesel would improve efficiency more (~40-50%?) because engine efficiency in a diesel would only decrease linearly with load. While in a gasser, he has to deal with the near exponential drop in efficiency compared to load. In the BSFC maps above, going from a third load to a sixth load decreases engine efficiency by ~40% for the gasser, so the driver may only see ~10-20% increase in fuel mileage even though they made the car itself twice as efficient. A diesel otoh, will see a relatively small drop in engine efficiency of ~15%, and can really reap the rewards of  making the car twice as efficient.

Lastly, as slow as possible in top gear nets the best mileage. I'm getting the same mileage in my ~185hp V6 automatic gasser that these posters are getting in their diesels because I cruise at ~55mph on the highway. As I go faster, my mileage goes down.

[jtanguay]

what rpm's are you seeing @ 55 mph??? fuel consumption is definitely increased as the rpm's rise. at 4000 rpm you're basically getting about 15-20 mpg less than usual.

[lyeinyoureye]

~2000rpm. If I keep on driving the car, I'll probably into a custom fifth gear in a manual trans so I could be at ~1000rpm at ~55mph. All the OEM manuals suck balls in terms of fuel efficiency, something like 700rpm or more than the 4spd automatic. Or, I may just drop in a 3L 4 cylinder diesel...  :lol:

[addautomotive]

Question for the 1.5 netsperts:

If the 1.5 & 1.6 bore is the same, can a 1.6 TD block be used with 1.5 crank & pistons to build up a proper 1.5td?