OT: Diesel compression, was: Range: 156Volts vs 312 volts

[cowtown]

>> Diesel powered cars have very little compression braking and seem to be
>> able to descend hills safely.
>
>    Huh? What? Diseasel cars compression brake just fine! Hell you  
> need a GP 27 battery to crank the damn things when it's cold! Of  
> course they go down hills great in low gear, like throwing out the  
> anchor!

Yeah - isn't that what a "Jake brake" on a big rig is used for? Noisy  
as hell and not legal inside city limits, but they put that 15-25:1  
compression to work as an air pump through the exhaust!

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[Peter VanDerWal]

Diesel engines have lots of Compression (hence the need for powerful
starting batteries) but this does NOT mean they have lots of compression
braking.
Most of the energy needed to compress the air in the cylinder gets
returned when the piston moves down (often called an Air Spring)

Having the throttle closed is what provides compression braking in a
gasoline powered car.  Diesel do NOT have a throttle flap to close off the
air intake.

A "Jake Brake" is an ADD-ON to a diesel engine so that big rigs can get
some compression braking.  The overwhelming majority of Diesel cars do NOT
have Jake Brakes.  For the most part they rely on mechanical brakes.

As for EVs, I do agree that the brakes of a gasoline powered car /may/
need some improvement if you go down any long, steep hills.  I'm just
pointing out that since diesel powered cars can get away with just
mechanical brakes, then electric powered ones can probably do the same.

I'm also pointing out that EVs do NOT generally go down long, steep hills,
because they generally can't climb long steep hills.

>>> Diesel powered cars have very little compression braking and seem to be
>>> able to descend hills safely.
>>
>>    Huh? What? Diseasel cars compression brake just fine! Hell you
>> need a GP 27 battery to crank the damn things when it's cold! Of
>> course they go down hills great in low gear, like throwing out the
>> anchor!
>
> Yeah - isn't that what a "Jake brake" on a big rig is used for? Noisy
> as hell and not legal inside city limits, but they put that 15-25:1
> compression to work as an air pump through the exhaust!
>
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[me2]

Diesel engines have very little compression braking compared to a gas
engine.

A gas engine has 14psi of vacuum on the piston as it goes through the
intake stroke.   A diesel engine has no such vacuum.   Both the gas and
diesel engine compress the air during the compression stroke, but most
of that energy is returned on the "combustion" stroke.  (There probably
is no combustion during engine braking on either engine as the fuel is
generally turned off entirely.)

Engine braking is a problem on diesel trucks, thus the use of exhaust or
Jake brakes.

Diesel engines need large batteries to power the starter because the
rotating mass is generally much larger and because of the energy
required to compress the air mass to the higher compression ratio.  This
has nothing to do with its braking potential.  High compression gasoline
engines need a more powerful starter/battery as well.  Each has a
different engine braking characteristic.


On Mon, 2008-05-12 at 23:47 -0700, cowtown@... wrote:

> >> Diesel powered cars have very little compression braking and seem to be
> >> able to descend hills safely.
> >
> >    Huh? What? Diseasel cars compression brake just fine! Hell you  
> > need a GP 27 battery to crank the damn things when it's cold! Of  
> > course they go down hills great in low gear, like throwing out the  
> > anchor!
>
> Yeah - isn't that what a "Jake brake" on a big rig is used for? Noisy  
> as hell and not legal inside city limits, but they put that 15-25:1  
> compression to work as an air pump through the exhaust!
>
> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev

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[Son of Tesla]

Hello,
   
  Sorry to continue this OT but I beg to differ here.
  Diesels do NOT have as much compression braking as gasoline engines do.
  That's why they ADD Jake brakes and  "Retarders" in Allison and other automatic Transmissions.
  Diesels are designed without an air "throttle" the intake air flow is unrestricted.
  In a gas engine when the throttle is closed / idled,  the restriction causes a compression and decompression imbalance within the cylinders in favor of a much quicker rpm reduction.
   
  In a diesel, no throttle means the measure of intake air ingested is equal to the max flow for that particular rpm.
  It would appear reasonable to assume that compressing all this air volume would take energy and that expended energy would decrease the RPM
  BUT
  while there is an increase in expended energy to compress the cold air charge it is offset by the now heated air which at full compression (piston @ TDC)  produces slightly more energy
  due to the thermal dynamics of "heat of compression" and the absorbed heat built up in the cylinder.
  The only reasons a diesel slows down at all without additional assistance, is due to
  1) The absorbed heat from the surrounding engine parts rapidly decreases due to the no fire condition that exists which tends to negate the "heat of compression" increase and
  2) Mechanical friction.
   
  Hence the need for the "Jake brake" and Allison's transmission feature the "Retarder"
  I'd explain these further but
  1) Enough OT and
  2) Full explanations of their respective operational theory is available readily online and in print.
   
  My.02 (OT as it is)
  Cruiser
   
 
cowtown@... wrote:
  >> Diesel powered cars have very little compression braking and seem to be
>> able to descend hills safely.
>
> Huh? What? Diseasel cars compression brake just fine! Hell you
> need a GP 27 battery to crank the damn things when it's cold! Of
> course they go down hills great in low gear, like throwing out the
> anchor!

Yeah - isn't that what a "Jake brake" on a big rig is used for? Noisy
as hell and not legal inside city limits, but they put that 15-25:1
compression to work as an air pump through the exhaust!

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[James Massey]

G'day Peter, All

At 05:04 AM 13/05/08 -0600, Peter VanDerWal wrote:
>Diesel engines have lots of Compression but this does NOT mean they have
>lots of compression
>braking.
><snip> Diesel do NOT have a throttle flap to close off the
>air intake.

Well, I don't know what diesels you've been driving but lets see : two
Isuzu sedans, Toyotal Landcruiser, a little Ford hatch, two Toyota trucks,
all been driven and maintained some by me over the last 20-odd years, ALL
with throttle bodies that produce great vacuum when closed (and almost none
when open, hence the alternator-mounted vacuum pumps).

I get asked by people who have followed me if there is something wrong with
my brake lights because "they hardly ever come on". So I explain how I
mostly brake using the engine, only using the mechanical brakes to stop, or
brake harder than the compression braking allows. The Fiat turbo-diesel EFI
van we recently got has a turbo, and it doesn't have the same engine
braking. So on my sample, most diesel engines have far greater compression
braking than a manual petrol engined vehicle.

As for automatic transmission vehicles - compression braking? well, I pull
back the gears to stop either of the Skylines' running away down the steep
hills, but it is nothing like what I get from the diesels, even the
turbo-diesel.

>A "Jake Brake" is an ADD-ON to a diesel engine so that big rigs can get
>some compression braking.  The overwhelming majority of Diesel cars do NOT
>have Jake Brakes.  For the most part they rely on mechanical brakes.

That is because a big truck when loaded weighs up to three times its'
unladed weight, so their brakes need to be able to operate successfully
over that range, a car on the other hand when fully loaded weighs about 1.3
times the unladen mass. An exhaust brake can let the pressure get many
times higher than the vacuum, so many more times the effect. The driver
controls wether he uses the exhaust brake or not, depending on the amount
of braking force he needs. I'm guessing you've not driven a vehicle with an
exhaust brake?

>As for EVs, I do agree that the brakes of a gasoline powered car /may/
>need some improvement if you go down any long, steep hills.

Just as your EV /may/ be heavier than its's pre-conversion weight? Unless
you have regen, you will not have the same effective braking capacity. If
you have an EV with regen, the regen should be able to be programmed to
give any value of braking effect. Without it you have a heavier vehicle
with less engine braking. If possible, an EVs brakes should be upgraded at
the same time as the suspension is upgraded, and by at least the same
factor, unless the vehicle had exceptionally good brakes for its type (e.g.
the vehicle was a turbo variant or similar 'sporty' type with bigger brakes
anyway).

>  I'm just
>pointing out that since diesel powered cars can get away with just
>mechanical brakes, then electric powered ones can probably do the same.

Sorry Peter, your datapoint is off.

Hope this helps someone, rather than wasting BW.

Regards

[Technik] James

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[Roger Stockton]

James Massey wrote:

> >As for EVs, I do agree that the brakes of a gasoline powered
> >car /may/ need some improvement if you go down any long,
> >steep hills.
>
> Just as your EV /may/ be heavier than its's pre-conversion
> weight?

Almost: I would suggest just as your EV /may/ exceed the original GVWR.

Diseasels aside, I think Peter's statement above really does succinctly capture the essential point.  In my experience, if your EV is within the original GVWR, then it is probable that you will not have to upgrade its brakes for typical driving conditions.  If, as Peter suggests, your driving includes descending long steep hills, then you may want/need to upgrade the brakes even if you are under GVWR.

Obviously, if your driving includes descending grades that *require* the use of engine braking to prevent the friction brakes from failing in an ICE, then you will need to upgrade the brakes in an EV that is to be driven on the same grade.  I personally would not count on regen as a substitute for adequately-sized mechanical brakes.

Cheers,

Roger.

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[robert harder]

> From: jcmassey@...
> Subject: Re: [EVDL] (Off Topic): Diesel compression, (on topic) EV  brakes
>
> G'day Peter, All
>
> At 05:04 AM 13/05/08 -0600, Peter VanDerWal wrote:
> >Diesel engines have lots of Compression but this does NOT mean they have
> >lots of compression
> >braking.
> ><snip> Diesel do NOT have a throttle flap to close off the
> >air intake.
>
>> Well, I don't know what diesels you've been driving but lets see : two
>> Isuzu sedans, Toyotal Landcruiser, a little Ford hatch, two Toyota trucks,
>> all been driven and maintained some by me over the last 20-odd years, ALL
>> with throttle bodies that produce great vacuum when closed (and almost none
>> when open, hence the alternator-mounted vacuum pumps).
>

Umm, sorry but,  diesels have no throttle bodies to close and do NOT produce vacuum, they are allowed all the air they can get at all times and vary only the fuel in order to vary power, that having been said my diesel (jetta) will put you through the windshield under compression braking, but will not hold itself in place when parked via compression, and I have to say no conversion should be relying totally on the vehicle service brakes they are prone to total failure without warning, and without some form of resistance braking at least for emergencies I belive you are asking for trouble

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[James Massey]

At 02:34 AM 16/05/08 +0000, robert harder wrote:

> > From: jcmassey@...
> > Subject: Re: [EVDL] (Off Topic): Diesel compression, (on topic) EV  brakes
> >
> > G'day Peter, All
> >
> > At 05:04 AM 13/05/08 -0600, Peter VanDerWal wrote:
> > >Diesel engines have lots of Compression but this does NOT mean they have
> > >lots of compression
> > >braking.
> > ><snip> Diesel do NOT have a throttle flap to close off the
> > >air intake.
> >
> >> Well, I don't know what diesels you've been driving but lets see : two
> >> Isuzu sedans, Toyotal Landcruiser, a little Ford hatch, two Toyota
> trucks,
> >> all been driven and maintained some by me over the last 20-odd years, ALL
> >> with throttle bodies that produce great vacuum when closed (and almost
> none
> >> when open, hence the alternator-mounted vacuum pumps).
>
>Umm, sorry but,  diesels have no throttle bodies to close

G'day All

Funnily enough, as I said off-list to Peter, the absolute answer is "it
depends". For my sample of vehicles, most have throttle bodies and do
produce vacuum. Definately, absolutely certainly do have them. Large
vehicle diesels are out of context as conversion candidates, and so what
they do or do not have is not relevant. Other diesel engines in potential
conversion candidate sized vehicles may or may not have them and would be
very unlikely to have exhaust brakes. My parent's Toyota Land Cruiser (long
since sold) was non-turbo diesel and had a throttle body, and I believe the
turbo variants of that vehicle also had throttle bodies. My most recent
vehicles (Fiat Ducato trucks) are the first diesels that I have had that
are EFI turbo-diesels and I don't know if they have throttle bodies or not,
but I suspect not.

Also, *in my experience* the worst compression braking is from the variant
of anything with an automatic transmission (regardless of diesel or
petrol). Your results may vary, as there are a great variety of vehicles
out there. However, *from my experience* I would expect to find the biggest
brakes of a particular variety of passenger vehicle on a turbocharged [or
whatever is the most powerful] automatic transmission (weakest effective
compression braking) variant. Diesels tend to be heavier than petrol
engines, so despite the increased compression braking would be unlikely to
have smaller brakes.

My first employer had two Holden Gemini (Isuzu) diesel sedans. On several
occasions (until I got used to them) I spun them in the wet by lifting off
the accelerator and hitting the brakes without de-clutching. The effect of
compression braking plus mechanical brakes would break rear traction to
cause the spin. The solution was to de-clutch when breaking in the wet.

Diesel engine variants may be better candidates for DC motor conversions,
as *the ones in my experience* have had different gear ratios due to
revving to lower maximum RPMs [your results may vary]. For a DC conversion
this may provide motor speeds that are in better ranges for the typical
use. Again, that would depend on the needs of the user.

I live in a hilly town, so really notice the difference in compression
braking levels between the different vehicles I drive (between my business
and family that is currently seven vehicles that cover the scope of the
discussion). Because of this I want to do something to gain some
regenerative braking in my EV-in-progress to make up for the loss of
compression braking. I can't afford AC at this time, plus have a good
collection of things suitable for a series DC conversion. Consequently the
regen braking will be from some kind of 'add on', which as a bonus will
give me something else to tinker with :^)

Hopefully this ends the discussion on diesel engines and returns focus to EVs.

Regards

[Technik] James

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[me2]

On Fri, 2008-05-16 at 18:00 +1000, James Massey wrote:

> Funnily enough, as I said off-list to Peter, the absolute answer is
> "it
> depends". For my sample of vehicles, most have throttle bodies and do
> produce vacuum. Definately, absolutely certainly do have them. Large
> vehicle diesels are out of context as conversion candidates, and so
> what
> they do or do not have is not relevant. Other diesel engines in
> potential
> conversion candidate sized vehicles may or may not have them and would
> be
> very unlikely to have exhaust brakes. My parent's Toyota Land Cruiser
> (long
> since sold) was non-turbo diesel and had a throttle body, and I
> believe the
> turbo variants of that vehicle also had throttle bodies.

My friend had a diesel Toyota Land Cruiser.  It didn't have a throttle
body.  I've never seen a diesel engine with a throttle body and I've
seen a lot of them.  If they do have a throttle body, what is it
connected to ?  The accelerator ?

The newest, cleanest diesels have a throttle body for emissions
purposes.  It creates a vacuum in the intake manifold so that exhaust
gasses can be drawn in for recirculation.

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