Brake specific fuel consumption

[muffildy]

So,
i had an interesting thought - and i wanted to know if anyone knew if it was implemented or not.
Brake specific fuel consumption for a gas or diesel engine usually has a 'sweet spot' where the torque and rpm use the least amount of fuel. Usually auto manfucturers will make a specific gear and a specific speed meet the sweet spot eg 65 mph 4th gear = sweet spot in theory, but with changing road conditions it might not be there, so....
I was wondering if any auto manufacturers of HEV use this to advantage in the recharging of the battery pack? If the car could read the current fuel consumption and rpm, it would know the minimum torque/max torque in order to stay in the sweet spot and then use the electric motor to charge the batteries at a rate which would increase the torque to that amount. it would theoretically keep an engine in its sweet spot almost all of the time.

[user removed]

Its done in some hybrids. Peak BSFC is more power than the car will use at any normal cruising speed. To reach peak BSFC you would have to reduce the engine size to the point where maximum power would be insufficient for climbing grades and other high sustained load situations.

regards
Mech

[JRMichler]

And then you spend a lot of time at WOT, where the efficiency is low.

[dcb]

curious, because most bsfc charts that I have seen show peak engine efficiency at or near WOT (though in the 2-3000 rpm range). Though if you have a torque converter that can compound losses at higher engine torque outputs. WOT can be inefficient also if you have to brake more.

To charge batteries then discharge them is a lossy proposal, I haven't seen huge gains in peak bsfc with an engine tuned to run at one rpm and load, not compared to the losses in a mechanical->electric charge->discharge->mechanical setup that I have seen.

re "sweet spot", the real sweet spot is peak bsfc, which has a load component and an rpm component. Lets say for example peak bsfc is at 2500 rpm at 80% throttle.

Most cars have far too much engine for optimal cruise efficiency, i.e. if you are making 40 hp at 2500 rpm, you would probably going to accelerate up to 100mph with enough gears if you left the throttle at %80. Even though your engine is close to peak bsfc, you are still making too much hp for a reasonable speed (say, 70mph)/ and a reasonably sized vehicle. If your power demands at 70 are, say 25 hp, then you need an engine that is ~%40 smaller to have peak bsfc at 70mph.

If you make aerodynamic or rolling resistance changes, then your constant cruise power demands go down even more.

[muffildy]

Well, a theoretical example:
compact car uses 13 hp to maintain 65 mph
the engine sweet spot is at 75 hp
so its producing 62 more hp at the sweet spot than it needs.
If you charge the batteries at 62 hp until they are full, then shut the engine off and run all EV i think you may get much better mpg.
So at 65 mph its getting an efficiency of say 20% since its not in the sweet spot, putting it in the sweet spot raises the engine to be 40% or so efficient.
13 hp x 400g/hp = 5200g; 3200g/gal = 1.625gal/hour 65 mph/1.625gal = 40 mpg which is around what the best fuel efficient cars get so these made up numbers arent too innaccurate.
Now 75hpx200g/hp = 15000g or 4.68gal/hour, if run for 30% of time, you would use 1.4gal, then 70% electric....well you saved (46kwh * 30%) * 80% efficiency =11kw if you get 250w/mi it would last you 44 mi, so 30% of 65 is 19mi + 44 mi = 63mi for 1.4 gal or 45 mpg that you went on that 1 gallon.

So 5 mpg better is what i might expect from this mod.

[some_other_dave]

Originally Posted by JRMichler

And then you spend a lot of time at WOT, where the efficiency is low.

I think most of the inefficiency at WOT is due to acceleration enrichment and going open-loop. If the engine management were set up correctly, you might even get better efficiency at WOT than at 80% load. There certainly would be less pumping loss past the throttle--not much less, but some.

It might at least be worth a try. Of course, you'd have to make sure you could distinguish between "going WOT for efficiency" versus "I want maximum acceleration now".

-soD

[busypaws]

Making the engine/computer/batteries do the same thing that we do now when the dirver Pulses and Glides. It would be P&G between engine and batteries and no loss in speed. So you could have the benefits of P&G without the loss in speed.
The problem I see is getting a big enough battery that could accept that 62hp of energy and then release it. Getting a big enough battery (today's battery technology) that could accept 62hp equivalent of energy and then release 13hp equivalent of energy to maintain speed in battery mode. That would add weight/size to vehicle and thus rolling resistance and aero drag.
A volt type Series hybrid would work better. You have the engine turn a generator at the "sweet spot" The engine generator is sized to only product the 13hp equivalent of long term driving. The battery is sized to allow any extra hp needed above 13 to be provided to the electric motor. So the battery managment would try and keep the battery at 60% charge. If the charge got downto 40% engine comes on runs at 13hp and recharges battery to 80% charge. You never charge the battery fully because then where does the energy go when you go down a hill or brake to stop. You never let the battery go completely empty or you won't have any power when you step on it to pass. Your always using your 13hp engine at it's sweet spot. When you get home your battery is between 40-80% charge, You do plug it in and top it off ot 80%.

This setup defeats the Volt purpose of driving 40miles on no engine running. Since we put a much smaller engine in the volts charging system it needs to cut on sooner so it stays ahead of any hills that might come up.

[JRMichler]

Originally Posted by Old Mechanic

Peak BSFC is more power than the car will use at any normal cruising speed. To reach peak BSFC you would have to reduce the engine size to the point where maximum power would be insufficient for climbing grades and other high sustained load situations.

regards
Mech

If you reduced the engine size that far, you would need to regularly run WOT up to redline RPM to climb hills and accelerate. BSFC drops at high RPM because of engine friction. The best overall compromise is an engine sized so that most driving is below best BSFC RPM.

[dcb]

Originally Posted by JRMichler

If you reduced the engine size that far, you would need to regularly run WOT up to redline RPM to climb hills and accelerate.

And this strategy works well for many people. My 125 is a good example, it is plenty fast around town, but can barely hold 55mph. When I do go 55, I am going to be there for a while, so more time at higher efficiency traded for a brief reduced efficiency rev. The alternatives are costly and less efficient.

[user removed]

Best BFSC can be easily determined with a vacuum gauge and a tachometer. If you match the dyno proven BSFC by duplicating the RPM and vacuum reading (or conversely the MAP reading) then you are at best BFSC.

It is never at WOT on any modern fuel injected vehicle engine. WOT is floored. If you have enough load on the engine you can get very close to 100% MAP and or 0 Inches of vacuum. It may actually happen at very low throttle position percentages depending on the load applied.

For those who can read vacuum or MAP just use a higher gear at lower speeds and gradually increase the throttle position until you reach full load. Do this at the lowest throttle position possible. When you get the same readings that are repeatable try a lower gear and see if you can do the same thing. The lowest gear that allows you to sustain those parameters will give you best acceleration for the least fuel.

To low a gear and you can not sustain the low vacuum. to high a gear and your acceleration will be to slow to get to the speed desired in a reasonable amount of time.
Find the correct balance and you can make your acceleration as efficient as possible.

I have seen my brothers Prius charging the battery under these circumstances, where the rate of acceleration or cruising would not be sufficient to maintain best bsfc. In those cases the system will charge the battery. Not every time, I guess it depends on the state of charge. I read about the Hyundai Sonata Hybrid doing the same thing.

I does allow some additional electric only operation of the vehicle, where the liquid fuel consumption would be 0.

regards
Mech