Newbie question: Why two batteries?
Freshman User
Joined: Nov 2010
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Yea man, I am wondering how the genius figures that it takes the same effort to crank lower compression engines. Not to mention that Diesel combustion takes place when the heat generated by molecular movement under high compression ignites the fuel and the valves are closed tight unlike gasser counterparts that can ignite slightly before or after the valves open (or close). That is why the diesel starter is so big, and why it takes so much current to turn it over, and why their are 2 batteries not one. I rest my case.
Junior User
Joined: Feb 2011
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That also brings up a good thought I had. Although my truck is basically brand new and only has about 3500 miles on it, I am already looking to the future and thinking about replacing my original batteries (after the fail) with a pair of those spiffy Optima batteries.
The only wrinkle being I had considered using one "red top" (traditional battery) and one "yellow top" (deep cycle) for extended engine-off use for various bits of electronics I have and would prefer not to idle to use.
Will that work using 2 slightly different battery styles?
Thanks!
-Rob
The only wrinkle being I had considered using one "red top" (traditional battery) and one "yellow top" (deep cycle) for extended engine-off use for various bits of electronics I have and would prefer not to idle to use.
Will that work using 2 slightly different battery styles?
Thanks!
-Rob
Super Moderator
Joined: Aug 2003
Posts: 25,479
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From: Isanti, MN
You can think what you like, I lose nothing by not convinving you or anyone else. But I will not call you names for not agreeing with me.
Elder User
Joined: Jul 2010
Posts: 642
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From: herriman utah
no such thing as decompressing, if a cylinder is going up its either compressing or pushing out exhaust, if its going down its either drawing in air/fuel or on its power stroke, if there is no power yet in the engine you have nothing to assit the power stroke in turning over the engine, but you do have a compressing force that is being created. and thus more power required to create it.
the main reason for 2 batteries is the glow plugs draw a TON of power about the same as turning the starter over. my old '93 dodge only came with one battery albeit about the biggest one I have ever seen in a pickup, anyways when the grid heaters would cycle there was a substantial dimming of the head lights and blower fan inside the cab would slow. yeah it worked ok but I could see where many who didn't know better would think there was something wrong with the truck.
the main reason for 2 batteries is the glow plugs draw a TON of power about the same as turning the starter over. my old '93 dodge only came with one battery albeit about the biggest one I have ever seen in a pickup, anyways when the grid heaters would cycle there was a substantial dimming of the head lights and blower fan inside the cab would slow. yeah it worked ok but I could see where many who didn't know better would think there was something wrong with the truck.
Freshman User
Joined: Nov 2010
Posts: 47
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All valid reasons for two batts as far as I am concerned. Have 4 in my boat. Today batts are going to last abouut 4 years. Try to push the number and your reliability goes to pot. Best to figure betwen 4 and 5 years to just replace as a regular maintinance effort.
Freshman User
Joined: Nov 2010
Posts: 47
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Deep Cycle vs Starting Batteries.
That also brings up a good thought I had. Although my truck is basically brand new and only has about 3500 miles on it, I am already looking to the future and thinking about replacing my original batteries (after the fail) with a pair of those spiffy Optima batteries.
The only wrinkle being I had considered using one "red top" (traditional battery) and one "yellow top" (deep cycle) for extended engine-off use for various bits of electronics I have and would prefer not to idle to use.
Will that work using 2 slightly different battery styles?
Thanks!
-Rob
The only wrinkle being I had considered using one "red top" (traditional battery) and one "yellow top" (deep cycle) for extended engine-off use for various bits of electronics I have and would prefer not to idle to use.
Will that work using 2 slightly different battery styles?
Thanks!
-Rob
Starting batteries are designed to provide a large amount of current for a short period where a deep cycle battery is designed to provide steady current for a long period of time. The Deep cycle will sacrifice starting current or CCA in order to hold reserve power for running accessories. The best approach is to replace base on OEM recommendations. More CCA is always the general upgrade for automotive batteries Since the engineering has already been done by the manufacturer there is really no need to re-engineer as long as starting power was adiquate to begin with. Lead acid batteries today are going to last about 4 years (much longer if you can get silver oxide batts) so the safest no-brainer is to replace at 4 with the OEM recommended battery and a few more CCA's as long as you can keep the size the same.
Cargo Master
Joined: Dec 2007
Posts: 2,196
Likes: 120
From: Carstairs Alberta
well my reference is an old 6.2 chevy diesel, good batteries, no electronics to pull power, and warm engine so no glow plugs firing(had em on a manual switch anyways). ran out of fuel(bad tank selector switch)2 big batteries you would be amazed at how short a time it cranked before it started to majorly slow down. so yeah i too will go with it take more power to turn over. even back in the 70's and 80's the aftermarket shops were selling high power starter for worked up high compression gas engines. with the increase in compression they turned slowly with the factory starters
Super Moderator
Joined: Aug 2003
Posts: 25,479
Likes: 742
From: Isanti, MN
I was going to stay out of this, but I just can't help it.
The problem here is all the pressure the compression builds and where it goes.
Perhaps someone here can explain to me how a compression brake like a Jake Brake works?
So a Jake Brake pops the exhaust brake at TDC. Which prevents the pressurized gas from forcing the piston down again, which would provide virtually NOTHING for braking force.
I still have my Class A CDL, and have experience driving heavy trucks with these brake systems. When the Jake was turned off that big, high compression, 15.2L CAT engine provided virtually nothing for compression braking until the jake brake was engaged. Even when bobtail at 18,000 lbs there was virtually no engine braking. Compression alone does not provide more resistance on a spinning engine.
no such thing as decompressing, if a cylinder is going up its either compressing or pushing out exhaust, if its going down its either drawing in air/fuel or on its power stroke, if there is no power yet in the engine you have nothing to assit the power stroke in turning over the engine, but you do have a compressing force that is being created. and thus more power required to create it.
Perhaps someone here can explain to me how a compression brake like a Jake Brake works?
Originally Posted by Wikipedia
When the driver releases the accelerator on a moving vehicle powered by a diesel engine, the vehicle's forward inertia continues to turn the engine's crankshaft, drawing air into the cylinders as the pistons move down and compressing that air as the pistons move back up. The pressure of the compressed air pushes back on the up-going piston, tending to slow the vehicle.
But, without a compression release mechanism, as the piston passes through top dead center and heads back down again, the compressed air in the cylinder acts as a spring and pushes the piston down, returning most of the work done in compression back to the crankshaft, tending to accelerate the vehicle. The net effect is that the engine turns freely and the vehicle coasts.
When a compression release engine brake is active, a valve releases the pressure from the cylinder before the piston starts back down, so the slowing effect is present on the up stroke, but no accelerating effect is present on the down stroke and the net effect is the vehicle slows down.
With a gasoline engine, the mechanics are different and a special valve is not necessary for engine braking to happen when the driver releases the accelerator. In the gasoline engine, with the accelerator released, a throttle prevents the free flow of air into the cylinders, so there is little pressure to release at the top of the compression stroke. The throttle itself provides engine braking through friction in the air flowing through it. But a diesel engine does not have a butterfly valve to limit air on the intake side.
A compression release engine brake uses an extra lobe on the camshaft to open a second exhaust valve at the top of the compression stroke. The stem of this valve telescopes during normal operation so the valve remains closed, but is locked at full length by a solenoid when the engine brake is engaged so that the valve opens as directed by the cam. This releases the compressed air in the cylinder as described above.
The driver controls consist of an on/off switch and, sometimes, a multi-position switch that controls the number of cylinders on which the brake is active. When the driver has turned on the compression release engine brake, it will activate when the driver releases the accelerator. There are also switches on the clutch and accelerator pedals that deactivate the compression brake when the driver disengages the clutch or presses the accelerator.
But, without a compression release mechanism, as the piston passes through top dead center and heads back down again, the compressed air in the cylinder acts as a spring and pushes the piston down, returning most of the work done in compression back to the crankshaft, tending to accelerate the vehicle. The net effect is that the engine turns freely and the vehicle coasts.
When a compression release engine brake is active, a valve releases the pressure from the cylinder before the piston starts back down, so the slowing effect is present on the up stroke, but no accelerating effect is present on the down stroke and the net effect is the vehicle slows down.
With a gasoline engine, the mechanics are different and a special valve is not necessary for engine braking to happen when the driver releases the accelerator. In the gasoline engine, with the accelerator released, a throttle prevents the free flow of air into the cylinders, so there is little pressure to release at the top of the compression stroke. The throttle itself provides engine braking through friction in the air flowing through it. But a diesel engine does not have a butterfly valve to limit air on the intake side.
A compression release engine brake uses an extra lobe on the camshaft to open a second exhaust valve at the top of the compression stroke. The stem of this valve telescopes during normal operation so the valve remains closed, but is locked at full length by a solenoid when the engine brake is engaged so that the valve opens as directed by the cam. This releases the compressed air in the cylinder as described above.
The driver controls consist of an on/off switch and, sometimes, a multi-position switch that controls the number of cylinders on which the brake is active. When the driver has turned on the compression release engine brake, it will activate when the driver releases the accelerator. There are also switches on the clutch and accelerator pedals that deactivate the compression brake when the driver disengages the clutch or presses the accelerator.
Originally Posted by Jacobs Vehicle Systems
When activated, the Jacobs Engine Brake® opens the exhaust valves near the top of the compression stroke, releasing the highly compressed air through the exhaust system. Little energy is returned to the piston, and as the cycle repeats, the energy of the trucks forward motion is dissipated, causing the truck to slow down.
I still have my Class A CDL, and have experience driving heavy trucks with these brake systems. When the Jake was turned off that big, high compression, 15.2L CAT engine provided virtually nothing for compression braking until the jake brake was engaged. Even when bobtail at 18,000 lbs there was virtually no engine braking. Compression alone does not provide more resistance on a spinning engine.
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Post Fiend
Joined: Jan 2009
Posts: 6,521
Likes: 11
From: Greenwood, SC
I dunno, when you think about it, with no combustion during startup, the piston compresses air, no fire, then goes down for the power stroke but no valves have opened yet. Cylinder remains sealed until the power stroke is completed, then the exhaust valve opens on the next up stroke. Right?
Doesn't this mean that the energy stored with the compressed air that did not ignite gets transferred to the downstroke, while other cylinders are in the process of compressing?
The timing of these events definitely not going to make the motor easy to turn, but certainly would smooth it out to some degree? Technically it is 1 and maybe half of another cylinder decompressing while another 2 are compressing, something like that.
I dunno, not that big of a deal but just thinking it through.
Doesn't this mean that the energy stored with the compressed air that did not ignite gets transferred to the downstroke, while other cylinders are in the process of compressing?
The timing of these events definitely not going to make the motor easy to turn, but certainly would smooth it out to some degree? Technically it is 1 and maybe half of another cylinder decompressing while another 2 are compressing, something like that.
I dunno, not that big of a deal but just thinking it through.
Super Moderator
Joined: Aug 2003
Posts: 25,479
Likes: 742
From: Isanti, MN
Haha, that was good timing, Joel!
If that were the case at some point you'd have two decompressing for one compressing. But on a V8 you have 8 power strokes for every four revolutions, so I'm not sure about a situation like that, it would depend on the geometry of the crank and firing order.
If that were the case at some point you'd have two decompressing for one compressing. But on a V8 you have 8 power strokes for every four revolutions, so I'm not sure about a situation like that, it would depend on the geometry of the crank and firing order.
Hotshot
Joined: Nov 2000
Posts: 11,591
Likes: 14
From: Pflugerville, tx
good description here http://www.svfd.net/SVFD%20Files/Art...a%20diesel.pdf
but net, they turn off the power stroke, by releasing the compressed air just before ignition WOULD occur.. they have turned off the fuel too.. so the power to compress the air is used to brake.
so, this
1. turns off fuel injection
2. opens exhaust valve at the top of compression stroke instead of later on the exhaust stroke.
now, you can figure out all kinds of ways to make that happen with your hardware.. (sort of requires variable valve timing to be 100% accurate)
Sam
but net, they turn off the power stroke, by releasing the compressed air just before ignition WOULD occur.. they have turned off the fuel too.. so the power to compress the air is used to brake.
so, this
1. turns off fuel injection
2. opens exhaust valve at the top of compression stroke instead of later on the exhaust stroke.
now, you can figure out all kinds of ways to make that happen with your hardware.. (sort of requires variable valve timing to be 100% accurate)
Sam
More Turbo
Joined: Aug 2002
Posts: 538
Likes: 45
From: Lansing, MI
I dunno, when you think about it, with no combustion during startup, the piston compresses air, no fire, then goes down for the power stroke but no valves have opened yet. Cylinder remains sealed until the power stroke is completed, then the exhaust valve opens on the next up stroke. Right?
Doesn't this mean that the energy stored with the compressed air that did not ignite gets transferred to the downstroke, while other cylinders are in the process of compressing?
Doesn't this mean that the energy stored with the compressed air that did not ignite gets transferred to the downstroke, while other cylinders are in the process of compressing?
The problem is that when the first few pistons are compressing air the pistons on the downstroke haven't compressed any air yet and aren't helping so amount of effort to start the crankshaft rotating is higher with a high compression engine. One a few compression strokes have completed and those pistons can assist on the downstroke the effort required to spin the crankshaft will be a little easier.