Running an engine at lean conditions equals...

[natto fire]

Hey,

This is kind of a physical/philosophical question.

When I drive to work, there is a large hill that I usually drive down in third gear. While going down this hill, RPMs are around 3K and I typically let off the accelerator during 80+% of the hill. I know that having a lean mixture and just compressing air makes a lot of heat, but how much is actually absorbed by the water jacket. (My engine has a floating piston arrangement, and the sleeves are not very thick)

So yeah, the whole fuel economy thing while in gear has been shot to death, but I think my way is slightly efficient because the thermal energy that would have been wasted in the braking system is now being used to warm up the coolant (which is cool from just starting my drive) in the engine. However, this could be easily proved wrong by observing all of the thermal energy escaping from the exhaust.

I guess I am wondering how much of that energy is transferred to the coolant while in vacuum. As far as observation, it seems too close to measure, with the temp gauge reaching op. temp around the same time, depending on lights. (My drive is approximately 5 minutes on local roads, until I get on the freeway, in which my temp is up to about 1/2 of op. temp, then the last 4th gear pull to freeway speed brings it the full way to op. temp)

As far as variables, I have a new radiator cap, thermostat, and coolant. Hill is about a 7% grade for about 1/4 mile. I was also wondering how different compression ratios might affect this as well.

 

[The Boston Dangler]

i really doubt engine braking down the hill makes much impact on engine temps, as compared to driving on a level road, especially for a quarter mile. the block absorb and release heat at it's own rate, as will the coolant and radiator. you say 3000 rpm, but what does that mean? is your redline 4000? 10,000?

honestly, efficiently getting the coolant up to temp is irrelevant. what is relevant is getting the oil and catalytic converters up to temp, which will happen in just a couple minutes no matter what you do. you do know that your temp gauges are full of crap anyway, right?

 

[natto fire]

Well, I had my doubts as well, which is why I posted this thread. My redline is 7600 RPM, but like you say, that is irrelevant, as I know I am drawing full vacuum coasting down the hill. I don't care about other heat losses (such as the block radiating heat, which is what I am assuming you mean by "block absorb and release heat" because if you think my scenario is irrelevant, radiant heat losses through the block are still on another level of insignificance.

For the record, catalytic light-off time is pretty much irrelevant to fuel economy, and oil and coolant temps can be synonymous depending on how your engine is set up.

Yes, I know my temp gauge only shows the temperature of the coolant (or crap, as you call it) however, I have owned a couple cars with oil/coolant heat exchangers, which makes that statement pretty much null and void. I do realize oil temp is very important for frictional losses, which is why I added this tidbit that there are plenty of engines that have warm oil when the temp gauge shows normal op. temp.

 

[Zenmervolt]

You aren't "running lean" when you're engine-braking down a hill. You're simply not running at all. The ECU completely shuts off fuel flow to the injectors when you are engine braking. No fuel is being ignited at all. So the engine is actually producing less heat since it is not experiencing combustion. Most of the heat in an engine comes from the combustion of fuel. The heat generated by compressing the air is a minor contributor.

ZV

 

[The Boston Dangler]

i didn't realize fuel milage was your concern with the OP. i thought you were speaking of general energy transfer eficiency. by crap, i meant the temp gauges don't reflect any particular temperature, and most often will end up pointing in the middle to make the driver happy. every car i've driven had the temp needles right in the middle nearly all year long. i really doubt all those cars governed temps perfectly, regardless of ambient temperatures and traffic jams. for that matter, only once (a rented chevy van, early 2000's) did the oil pressure gauge fluctuate with normal driving.

 

[The Boston Dangler]

Originally posted by: Zenmervolt
You aren't "running lean" when you're engine-braking down a hill. You're simply not running at all. The ECU completely shuts off fuel flow to the injectors when you are engine braking. No fuel is being ignited at all. So the engine is actually producing less heat since it is not experiencing combustion. Most of the heat in an engine comes from the combustion of fuel. The heat generated by compressing the air is a minor contributor.

ZV

i disagree with that. i certain my car is still burning gas when i engine brake.

 

[Zenmervolt]

Originally posted by: The Boston Dangler

Originally posted by: Zenmervolt
You aren't "running lean" when you're engine-braking down a hill. You're simply not running at all. The ECU completely shuts off fuel flow to the injectors when you are engine braking. No fuel is being ignited at all. So the engine is actually producing less heat since it is not experiencing combustion. Most of the heat in an engine comes from the combustion of fuel. The heat generated by compressing the air is a minor contributor.

ZV

i disagree with that. i certain my car is still burning gas when i engine brake.

Carbureted or a Wankel then? Otto-cycle engines with EFI absolutely do shut down the injectors when the throttle is closed and the engine is above XXXX RPM (where "XXXX" is an arbitrary RPM chosen by manufacturers, typically between 1,000 and 1,500 RPM). This has been a universal practice among manufacturers since the late 1980's.

ZV

 

[LTC8K6]

I disagree about the gauges. In the cars I have owned that had temperatures on the gauges, the gauges accurately reflected the coolant temp, and that temp was always very close to the thermostat's rating.

Could the oil pressure have been regulated in that Chevy van?

In a modern EFI engine, you should not be burning fuel when engine braking.

 

[natto fire]

Originally posted by: Zenmervolt
You aren't "running lean" when you're engine-braking down a hill. You're simply not running at all. The ECU completely shuts off fuel flow to the injectors when you are engine braking. No fuel is being ignited at all. So the engine is actually producing less heat since it is not experiencing combustion. Most of the heat in an engine comes from the combustion of fuel. The heat generated by compressing the air is a minor contributor.

ZV

Yes. I already knew that, and was on your side for the whole "wars" that thread(s) started. I guess I should have been more clear on my definition of "lean mixture". Although the injector pulse width is next to nothing, there is still a mixture of air and fuel particles. It is very lean in that there are barely any fuel particles in ratio to the air particles.

The definition "compressing air is a minor contributor" does not really answer my question directly, but I guess a vague answer is better than being explained things I already know about.

 

[iamwiz82]

Originally posted by: Captain Howdy

Originally posted by: Zenmervolt
You aren't "running lean" when you're engine-braking down a hill. You're simply not running at all. The ECU completely shuts off fuel flow to the injectors when you are engine braking. No fuel is being ignited at all. So the engine is actually producing less heat since it is not experiencing combustion. Most of the heat in an engine comes from the combustion of fuel. The heat generated by compressing the air is a minor contributor.

ZV

Yes. I already knew that, and was on your side for the whole "wars" that thread(s) started. I guess I should have been more clear on my definition of "lean mixture". Although the injector pulse width is next to nothing, there is still a mixture of air and fuel particles. It is very lean in that there are barely any fuel particles in ratio to the air particles.

The definition "compressing air is a minor contributor" does not really answer my question directly, but I guess a vague answer is better than being explained things I already know about.

If this were really an issue would you not see grenaded engines sitting at the bottom of large hills or in the mountains?

Don't believe us, get an EGT sensor and watch for yourself.

 

[LTC8K6]

Running an engine at lean conditions equals...

Holes in pistons.

 

[SpeedEng66]

Originally posted by: LTC8K6

Running an engine at lean conditions equals...

Holes in pistons.

well first knocking would kick in

 

[Uhtrinity]

Originally posted by: SpeedEng66

Originally posted by: LTC8K6

Running an engine at lean conditions equals...

Holes in pistons.

well first knocking would kick in

Originally posted by: LTC8K6

Running an engine at lean conditions equals...

Holes in pistons.

Honda has produced engines with a lean burn mode since the mid ninety's. My Insight gets part of its efficiency from lean burn.

 

[Zenmervolt]

Originally posted by: Captain Howdy

Originally posted by: Zenmervolt
You aren't "running lean" when you're engine-braking down a hill. You're simply not running at all. The ECU completely shuts off fuel flow to the injectors when you are engine braking. No fuel is being ignited at all. So the engine is actually producing less heat since it is not experiencing combustion. Most of the heat in an engine comes from the combustion of fuel. The heat generated by compressing the air is a minor contributor.

ZV

Yes. I already knew that, and was on your side for the whole "wars" that thread(s) started. I guess I should have been more clear on my definition of "lean mixture". Although the injector pulse width is next to nothing, there is still a mixture of air and fuel particles. It is very lean in that there are barely any fuel particles in ratio to the air particles.

The definition "compressing air is a minor contributor" does not really answer my question directly, but I guess a vague answer is better than being explained things I already know about.

Sorry, wasn't intending to bring that whole question up again. Even with whatever minor amount of fuel may be residually in the cylinder, it's below the concentration necessary for combustion, that is, it's too lean to actually explode.

That would leave the heat generated being a function of the uncompressed volume of air sucked into the engine and the compressed volume, with additive minor effects of friction, then you'd need to take into account the thermal conductivity of the alloy making up the cylinder walls (as well as accounting for whatever heat is absorbed by the piston itself). Which is an awful lot of work to arrive at a small number.

I suppose that you could get close by measuring the downhill acceleration rate when coasting in neutral, comparing it to the acceleration rate while engine braking, finding out what the magnitude of the deceleration vector is during engine braking and then using that to figure out that force and then convert that into heat energy units somehow...

Anyway, it's something that is complex enough that I really don't want to bother with equations.

ZV

 

[LTC8K6]

Honda has produced engines with a lean burn mode since the mid ninety's. My Insight gets part of its efficiency from lean burn.

Chrysler was using lean burn long before that. What has that got to do with my joke about running too lean and holing a piston?

 

[Uhtrinity]

Originally posted by: LTC8K6

Honda has produced engines with a lean burn mode since the mid ninety's. My Insight gets part of its efficiency from lean burn.

Chrysler was using lean burn long before that. What has that got to do with my joke about running too lean and holing a piston?

Just pointing out that lean burn technology exists.

 

[alpineranger]

Thinking about energy used to heat the coolant is not the right way to think about this. The coolant is just there to prevent various metal parts from getting too hot. When they do, they'll be more prone to 1) promote preignition, and 2) get damaged. Now granted, it's important to get your engine to proper operating temperature, but the coolant temperature is only the indicator of this. What happens when the engine gets warmed up? The most important things are: 1) the catalyst functions properly, 2) the oxygen sensors function properly, 3) the rings will seal better, 4) the oil viscocity is optimum. Now, of these, 1 and 2 have no relationship to the coolant temperature at all, and 4 does not track very well with coolant temp. So, whether you're pumping enough heat into the coolant is besides the point.

Some other things to think about... When you're engine braking, you'll have the throttle pretty much closed, which yields very high manifold vacuum, and low combustion chamber pressure. Thus, the amount of air, and therefore fuel (even assuming no fuel cutoff), and therefore energy is low. Add in the practical considerations of fuel cutoff and EGR operation (assuming you have this, you may not), and you'll see that you really aren't heating anything up very effectively just by engine braking.