Throttle Body Injection, or Multi-Point Injection for fuel economy? 1

[REDlinesDOTcom]

Based upon the question in the subject, which fuel injection arrangement would you expect to yield best results for fuel efficiency?

I'm going to leave this open for discussion, then I'll post the results that I have come across on more than one occasion.

 

[patdaly]

Port, but only if they do not raise the Hp when changing injection arrangements.

They invariably do though.

 

[TStaples]

Assuming that by multi-point you are actually referring to Port Fuel Injection (PFI), and not including direct injection (which technically is also multi-point,) and also assuming that both engines intake systems are optimized for each type of injection system, then PFI wins. It is much easier to design a low restriction intake system, thereby improving economy for the same power levels, with PFI than with TBI. Wall wetting, and generating even cylinder to cylinder distribution are known problems that affect fuel economy (and driveability) on TBI vs. PFI.

TBI has only one benefit over PFI, it is somewhat simpler and cheaper, in high volume, than Port Injection. Patdaly is also correct. PFI opens up opportunities to make more HP, again due to the lower restriction intake systems, and it gives the engine designer flexibility to improve power and torque with swirl control plates, plenum tuning valves, variable runner lengths, etc . . ..

-Tony Staples

http://www.tscombustion.com

 

[REDlinesDOTcom]

Well, I can see how MPFI could make it easier to achieve a higher HP figure, but assuming one were to never go WOT and keep it to a light cruise for the best fuel efficiency, I have read numerous times (even on here at least once) that a MPFI system would ALWAYS provide better fuel economy over a TBI setup.

About 10 years ago I had a 1991 Honda Civic Si with the MPFI and recall stating to a friend who had a DX model of the same car (automatic on top of the TBI system he had) that I had gotten 350 miles out of the 11 gallon (31.8mpg) tank. He was not impressed, which surprised me figuring that he could not get any better than that. I don't recall what he stated that he was getting out of each tank, but I now own a 1991 Civic DX and can vouch that on the first complete tank I got 436 miles out of the tank. That's 39.6mpg...nearly 8mpg better on a setup that I have always heard could never be as fuel efficient as an MPFI.

I feel that the Si engine and DX engine are so closely related that they make a good comparison between the two fuel delivery systems. The Si is a 1.6 with a slightly larger stroke over the 1.5 in the DX. The cylinder head is the same casting even.

 

[TStaples]

Red,

There are too many differences between those two cars to list them all. Here are a few. The 1.6 Si engine is not optimized for economy, it is optimized for higher performance, compared to the DX engine of that vintage. The other thing you are ignoring is that the Si's came with higher performance rubber, and the DX's came with fuel economy special tires, which can be 2 mpg difference with ease. Final drive and transmission gear ratios were different. Etc, etc, etc . . ..

As Pat stated, it seems no engine designer ever does a PFI system without also bumping the performance. It's relatively easy to get more HP and an efficiency improvement!

-Tony Staples

http://www.tscombustion.com

 

[REDlinesDOTcom]

Well, at the time with my Si, I had the same size steel wheel as I do now on the DX. Brand of tires I do not recall, I did not have the Si wheels.

Final drive on the Si appears to be 4.25 vs. the DX having a 3.89 with 1-5 gear ratios being the same. I still do not agree with how the MPFI (PFI) can be claimed as being (or COULD BE, but they were not concerned with it being even equal) superior to TBI. A 0.36 FD ratio and POSSIBLY different rubber compounds cannot account for 7+ MPG difference on the DX.

 

[dgallup]

Since neither test used calibrated fuel measurements or distance measurements I rate this comparison right up there with most other urban legends. Neither gas pumps nor odometers are known for accuracy plus you changed the wheels. There are huge uncontrolled variables; summer vs winter(?), oxygenated fuel(?), aerodynamics, tire pressure, etc, etc. I agree wholly with Tony, MPFI is a significantly superior system which will yield a better fuel economy/performance/emissions envelope. Where the designer decides to place a given vehicle within the envelope will make a huge difference in the actual mileage.

 

[patprimmer]

Port injection delivers the same amount of fuel to each cylinder. Provided each cylinder consumes the same amount of air, the ports will give substantially more even a:f ratios across the engine.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[patprimmer]

Some after market systems allow you to fine tune each injector.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[REDlinesDOTcom]

Well, besides the body (hatchback) being identical I don't see how much of the aerodynamics could have varied by much.

Pat...which way are you leaning?

Within the next few months I'll be switch the system over to a later style ECM and have the ability to tune it infinitely on the fly with a wideband O2 sensor to log all data points with.

I'm sure I'll be able to tune the system for even better economy, but that will not answer the question of how good could the Port Injection (TBI) have been. Oh well.

 

[patprimmer]

If optimised for economy, the ports will be better, especially on a modern engine with well designed plastic manifold so that each cylinder flows the same air.

As others have said, your comparison is not valid due to the many uncontrolled variables and unknowns.

For instance, do both engines have the same compression ratio.

Do they have the same exhaust system.

Do they have the same amount of valve overlap.

Does the inlet manifold feed equal air to each cylinder.

Are both air conditioned.

If so, did both owners use the air at equal rates.

Did one driver mostly drive windows down.

Does one car have roof racks.

Was one given more highway use.

Were the loads similar.

Was the driving style similar.

Etc etc etc.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[REDlinesDOTcom]

For instance, do both engines have the same compression ratio.

Do they have the same exhaust system.

Do they have the same amount of valve overlap.

Does the inlet manifold feed equal air to each cylinder.
The Si uses a log-style manifold while the runner length appears to be nearly equal on the TBI.

Are both air conditioned.
YES

If so, did both owners use the air at equal rates.
Since the owner is myself in this case, and I live in the same area of Florida as I always have, usage would be similar.

Did one driver mostly drive windows down.
No...with crank windows and wind buffing with windows down, I rarely drop them more than a crack for ventilation.

Does one car have roof racks.
Neither have roof racks

Was one given more highway use.
No. In fact, that "BEST TANK" that I had with the Si was a 2-hour drive from a friend's place, 5 days of city driving and the same 2-hour drive back to that friend's place. This past full tank on the DX was about 80% city driving.

Were the loads similar.
As far as load carried, I keep my vehicles mostly unloaded.

Was the driving style similar.
I can't claim my driving style was identical back then as to how it is now, but given that my best tank while TRYING to get the best MPG number was 86 miles LESS than this recent tank in the DX, I don't think it varied that much.

 

[REDlinesDOTcom]

For instance, do both engines have the same compression ratio.
9.1:1 on the DX vs. 9.2:1 on the Si I believe.

Do they have the same exhaust system.
Quite certain that they do.

Do they have the same amount of valve overlap.
This I would have to check into.

 

[dgallup]

Honda built some special high mileage vehicles, I remember the ads but I don't know model numbers or years. They wanted to have the best fuel economy of any model on the market. As such, the cars were strippers to keep the weight down. Honda did a lot of other special items for fuel economy. Narrow high pressure tires, narrow crank bearings and removal of one of the piston rings for reduced friction are some of the things I remember. These may well have been TBI engines as anyone that concerned with fuel economy in the days of $1/gallon gas would have also wanted an inexpensive car.

 

[REDlinesDOTcom]

I also recall weighing the Si at 2000 pounds even. The DX I have now weighs in at 2160 pounds.

 

[dcasto]

How about direct injection like that on the FORD 2.4 liter as installed in a MazdaSpeed 6?

 

[REDlinesDOTcom]

Well, a DI conversion is something that not only was I not planning on, but also something that would be very costly. My experiment is to so what mechanical alterations I can make to the cylinder head (and possibly the intake as well) to increase the MPG figures and document the real-world results as accurately as possible.

I've obtained a spare cylinder head that I have started mocking up with clay to have a plan for where to weld into, followed by machining to the final design. Should be interesting.

 

[reidh]

RedLines,

I think your missing the point that Patprimmer, TStaples, and Patdaly are trying to make. Without a plan, experimental controls, etc. it is impossible to make a comparison between the two injection methods.

I helped my niece a few weeks ago with some homework she had in her science class regarding the scientific method, which is relevant in this discussion. Per wikipedia:

"The scientific method is based on gathering observable, empirical, measurable evidence, subject to the principles of reasoning"

http://en.wikipedia.org/wiki/Scientific_method

I think that if these general guidelines were followed, much of this discussion above could have been avoided. Your hypothesis of the greater fuel mileage should then have been tested in a controlled environment, after which a conclusion could have been made.

Reidh

 

[kenvlach]

Actually, the scientific study on the 1991 Honda Civic Si vs. DX was done years ago.

REDlinesDOTcom, Using advanced technology [Google], I did some basic research to save you some work and allow you to quit belaboring the point.
The 1.6 L Si is rated 108 hp & EPA estimated 30 mpg (combined city & highway).
The 1.5 L DX is rated 92 hp & EPA estimated 35 or 33 mpg, depending whether 4- or 5-speed. As the 4-speed got better highway mileage than the 5-speed, it would seem that the extra gearing was for performance rather than economy overdrive type. Details below.

Honda Civic, under Fourth generation (1988-1991)
"A 1500 cc 16-valve fuel injected engine giving 92 hp (68 kW) was used in the DX hatchback/sedan, new LX sedan and the American market wagon. The Si and four-wheel drive wagons were powered by a 1.6 L 16-valve engine, rated at 108 hp (81 kW)...All engines sold in the US were fuel injected. "

http://www.reference.com/browse/wiki/Honda_Civic

1991 Honda Civic EPA Gas Mileage Estimates, MPG (city) (hwy) (combined)
4 cyl, 1.5 L, Manual (4 sp), 33 37 35
4 cyl, 1.5 L, Manual (5 sp), 31 35 33
4 cyl, 1.6 L, Manual (5 sp), 28 32 30
[Automatic trans. (4 sp) results are about 3-4 MPG lower]

http://fueleconomy.gov/feg/findacar.htm

 

[REDlinesDOTcom]

Ok, I apologize for wasting everyone's time. What I was actually asking was:

"WHY would a TBI systems get better results than an MPFI system, when I've seen stated so often that the reverse should be the situation?"

So, simply the larger stroke and factory tuning within the ECU being biased toward performance robs it of (per the EPA) 3mpg and in my personal case nearly 8mpg.

My next goal is to see how much can I squeeze out of this vehicle. By means of combustion chamber redesign, compression ratio alteration, along with converting to MPFI and the ability to tune it like a standalone.

What do you guys think the ceiling would be?