dual alternators

BigToe wrote:
Dual alternators also serve another purpose in diesel engines: to protect the glow plugs from early burn out.

As diesel engines came aboard light trucks in the late 80's and early 90's, some ambulance package vehicles were fitted with 215 amp alternators in Fords. But by the late 90's, Ford abruptly stopped installing single high amp alternators, and instead installed two alternators.

The reason? Saving the glow plugs, due to steadily increasing emissions regulations that required the glow plugs to continue to burn for a couple of minutes AFTER the engine was already started. This afterburn time with the engine running created a new problem. High amp alternators could more than meet the electrical load demand, recharging the batteries after startup with as much current as the batteries could absorb, while maintaining full regulation set point voltage. This was a problem for glow plugs, especially earlier on trucks that do not have sophisticated PCM controlled alternators.

The heating element of the glow plug has a fixed resistance that does not change, even if the glow plug gets hotter and hotter. If resistance is fixed, then an increase in voltage will automatically cause an increase in current.

Believe it or not, most glow plugs are rated at around 11 volts. Some as little as 10.5v, and some are 11.5v. Glow plugs are designed to work at lower voltages, because their PRINCIPAL purpose is to do their job PRIOR to the engine starting, relaying solely on battery power alone.

If you hooked up a volt meter to your electrical system and turned the key on, but not starting the engine, on a cold morning, you might see only 10.5 volts, even if your batteries were new or freshly charged to 13v. The drop in voltage is due to the glow plugs operating prior to start. The current draw reduces the voltage, hence glow plugs are designed the operate at the voltage they will typically see, which is a volt or two less than 12v nominal.

But now start the engine... and the alternator kicks on, increasing system voltage toward the regulator set point, which might be 14.2v. With cold start emissions reduction mandates, the glow plugs do not shut off, but continue to burn for up to 2 or even 3 minutes in the latest models. Yet with the engine running, there is now 14 volts available to them, which increases the current the glow plugs will see, in the absence of any other intervening regulating device.

Fortunately, modern day glow plugs DO have a self regulating device in the form of a secondary pass thru resistor that IS temperature sensitive, and in fact has a very high thermal coefficient. As this pass thru resistor gets hotter, it's resistance increases, and the increase in resistance reduced the current flow, which counteracts the increase in voltage which would otherwise increase it.

Let's at this point establish that when glow plugs consume too much current, the tips can swell, making them impossible to remove without removing a head, and even worse, the tips can break off inside the cylinder, ruining a motor. We don't want glow plugs consuming any more current than necessary to get the engine started and meet cold start emissions.

Getting back to the dual lower power alternators, versus a single high power alternator, the single high power alternator that easily meets all the current demand of charging the potteries and continuing to power the glow plugs, even at idle, will maintain a higher system voltage. On the other hand, an underpowered alternator, that is anemic at idle, perhaps only capable of generating 50 amps at idle... cannot keep up with recharging the batteries and continuing to supply the glow plugs the current that the glow plugs can consume before the secondary pass through resistor that self regulates the glow plug gets hot enough to increase resistance and reduce current demand.

So this was how Ford started protecting the glow plugs in diesel engines... by fitting two alternators instead of one, and keeping the second alternator turned OFF during engine start, as well as anytime the PCM commanded the glow plugs to be on. Ford also issued letters to ambulance builders threatening to pull their Qualified Vehicle Modifier approval status if they tried to go back to their old ways of installing a single high amp alternator, or leaving the dual alternator set up but replacing the primary alternator with a higher amperage model.

Once the glow plugs turned off (depending on Engine Oil temperature, barometric pressure, a total on time limitation timer, and system voltage), the PCM went ahead an let the second alternator kick on and come on board supplying more electrical power to the system.

Of course, over the 19 years since Ford moved to dual alternators to save glow plugs, Ford has since moved to PCM controlled alternators, and three stage glow plugs with more reactive self regulation. But 20 years ago, saving the glow plugs (or rather, saving warranty engine replacements due to glow plug failure) played a significant part in the decision to go with two smaller alternators, and keeping one turned off during glow plug operation, in order to limit the electrically generated power available to the glow plugs as part of the regulation strategy.

Thanks for one of best explanations of something I have ever seen on the TC board..

Had/have two of em' also on all my trucks. If you order plow pkg. it's standard, along with the front stabilizer bar, even though my plowin days are over

On the Ford 6.7 diesel they work in parallel and cannot be easily separated. Aparently the computer is hunting for both of them.

I am highly mechanically-inclined and even I have degree in different majors -cars have been always my hobby and I am die-hard DIY in vehicle repairs.
Still when I open the hood of my new SuperDuty, I found heat exchanger that I could not identify. Later found on the net it is liquid-cooled intercooler.
In old days I would investigate such design, but now I rather enjoy my early retirement.
Truck has long warranties and MAYBE I will change oil on it, but don't plan to learn every switch under the hood.

Are you trying to say that people will not keep up with technology even if they are technology oriented themselves? I will agree with that statement if the person thinks they can learn once and not continue learning. In my job, it is constant education built on top of a large foundation of basics. The foundation doesn't really change with new technology, but the terminology is in constant churn even if the principle hasn't changed.

Let me rephrase it for you.
Technology change with the speed that even inclined people will loose the connection sooner or later.

Kayteg1 wrote:
Technology change the way that even inclined people will loose the connection sooner or later.

This phrase makes no sense at all.

Technology change the way that even inclined people will loose the connection sooner or later.
My 2007 Powerstroke had long-glowing plugs.
Even in California I had to sit few seconds before I could turn the engine over.
Than the computer would not activate alternator for a few minutes, probably to avoid plugs over-voltage.
But in times when I had to move the truck on the yard couple of times, it would not turn on the 3rd as the batteries never had recharge.
NO WAITING TIME FOR GLOWING ON MY 2017 POWERSTROKE
that things fires on instant on cold 50F mornings.
out of curiosity I need to hook up voltmeter to it.
Bad news, it sounds like gasoline Chevy 4-banger, but than it takes some serious reeving to hear it.
Don't ask me for pictures of dual alternators. I took a peak and you hardly see alternators buried behind water-cooled intercooler and other stuff.
Next time I plan to open the hood will be about 10,000 miles.

Wow, sure glad my diesel don't have glow plugs :)

Dual alternators also serve another purpose in diesel engines: to protect the glow plugs from early burn out.

As diesel engines came aboard light trucks in the late 80's and early 90's, some ambulance package vehicles were fitted with 215 amp alternators in Fords. But by the late 90's, Ford abruptly stopped installing single high amp alternators, and instead installed two alternators.

The reason? Saving the glow plugs, due to steadily increasing emissions regulations that required the glow plugs to continue to burn for a couple of minutes AFTER the engine was already started. This afterburn time with the engine running created a new problem. High amp alternators could more than meet the electrical load demand, recharging the batteries after startup with as much current as the batteries could absorb, while maintaining full regulation set point voltage. This was a problem for glow plugs, especially earlier on trucks that do not have sophisticated PCM controlled alternators.

The heating element of the glow plug has a fixed resistance that does not change, even if the glow plug gets hotter and hotter. If resistance is fixed, then an increase in voltage will automatically cause an increase in current.

Believe it or not, most glow plugs are rated at around 11 volts. Some as little as 10.5v, and some are 11.5v. Glow plugs are designed to work at lower voltages, because their PRINCIPAL purpose is to do their job PRIOR to the engine starting, relaying solely on battery power alone.

If you hooked up a volt meter to your electrical system and turned the key on, but not starting the engine, on a cold morning, you might see only 10.5 volts, even if your batteries were new or freshly charged to 13v. The drop in voltage is due to the glow plugs operating prior to start. The current draw reduces the voltage, hence glow plugs are designed the operate at the voltage they will typically see, which is a volt or two less than 12v nominal.

But now start the engine... and the alternator kicks on, increasing system voltage toward the regulator set point, which might be 14.2v. With cold start emissions reduction mandates, the glow plugs do not shut off, but continue to burn for up to 2 or even 3 minutes in the latest models. Yet with the engine running, there is now 14 volts available to them, which increases the current the glow plugs will see, in the absence of any other intervening regulating device.

Fortunately, modern day glow plugs DO have a self regulating device in the form of a secondary pass thru resistor that IS temperature sensitive, and in fact has a very high thermal coefficient. As this pass thru resistor gets hotter, it's resistance increases, and the increase in resistance reduced the current flow, which counteracts the increase in voltage which would otherwise increase it.

Let's at this point establish that when glow plugs consume too much current, the tips can swell, making them impossible to remove without removing a head, and even worse, the tips can break off inside the cylinder, ruining a motor. We don't want glow plugs consuming any more current than necessary to get the engine started and meet cold start emissions.

Getting back to the dual lower power alternators, versus a single high power alternator, the single high power alternator that easily meets all the current demand of charging the potteries and continuing to power the glow plugs, even at idle, will maintain a higher system voltage. On the other hand, an underpowered alternator, that is anemic at idle, perhaps only capable of generating 50 amps at idle... cannot keep up with recharging the batteries and continuing to supply the glow plugs the current that the glow plugs can consume before the secondary pass through resistor that self regulates the glow plug gets hot enough to increase resistance and reduce current demand.

So this was how Ford started protecting the glow plugs in diesel engines... by fitting two alternators instead of one, and keeping the second alternator turned OFF during engine start, as well as anytime the PCM commanded the glow plugs to be on. Ford also issued letters to ambulance builders threatening to pull their Qualified Vehicle Modifier approval status if they tried to go back to their old ways of installing a single high amp alternator, or leaving the dual alternator set up but replacing the primary alternator with a higher amperage model.

Once the glow plugs turned off (depending on Engine Oil temperature, barometric pressure, a total on time limitation timer, and system voltage), the PCM went ahead an let the second alternator kick on and come on board supplying more electrical power to the system.

Of course, over the 19 years since Ford moved to dual alternators to save glow plugs, Ford has since moved to PCM controlled alternators, and three stage glow plugs with more reactive self regulation. But 20 years ago, saving the glow plugs (or rather, saving warranty engine replacements due to glow plug failure) played a significant part in the decision to go with two smaller alternators, and keeping one turned off during glow plug operation, in order to limit the electrically generated power available to the glow plugs as part of the regulation strategy.