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Original article → The Exponential Engine
Author(s): Brian Bush
Published February 18, 2015 | Fire Engineering Blog

I would say there are two questions I field more than any others when it comes to fire streams and apparatus set up. The first is, “How is your engine set up?” and the second, “How would you set up an engine”. I believe most of the time people who ask the first question really want the answer to the second question. So rather than waste the explanation of how an engine is currently set up and what I would change I think it would be best to start with a blank sheet and explain one approach to setting up an a rig to maximize first due potential with the exponential engine approach.


This is intended to address the masses and focus on first arriving engine operations. Before it is taken further I will explain my observation and therefore the context of today’s most common engine company as it pertains to this piece.

  • 3 person staffing (Operator, Officer, Firefighter)
  • 2 person attack line (Officer, Firefighter)
  • Water as extinguishing agent (No CAFS option)
  • 500 gallon on-board tank
  • 1 ¾” , 2 ½” and Engine mounted master stream as initial attack options

Exponential Attack

The Exponential Engine (FE Blog Post) Comparison.jpg

In various firefighting and fire prevention documents you can find that given appropriate fuel and air a fire will double in size every XX seconds or minutes. I have seen it referenced as fast as 30 seconds and as long as 2 minutes. The difference in time of 30 seconds to 2 minutes has never bothered me too much as I see both as relatively fast, the point that always has stuck with me from that adage is the term “double in size”.

When I consider something doubling in size I think of exponential growth and I believe that if we view the fire as an enemy, exponential growth of the enemy’s force is a power curve that must be addressed swiftly and with dominance. I think that most engaged firefighters would agree with that point, but how we attack swiftly and with dominance has many forms when it comes to fire streams.

There are firefighters pushing for greater volume on all initial lines with the use of intermediate lines and tips like 15/16” and 1” tips on 2” attack lines. Others are advocating for immediate fire stream application for the exterior if there is an opportunity with an interior follow up to “reset the fire” and interrupt that exponential growth. All of these ideas have merit as they are people attempting to find the right solution for their agencies to address the modern fire environment. Among these ideas I would like to present one more way to combat exponential fire growth and that is with an exponential fire attack plan.

The exponential engine idea came to me while I was sitting in a class at FDIC being delivered by Chief Curt Isakason from Escambia County Fire Rescue. Chief Isakson was speaking about the importance of rapid water application and he instantly shifted my thinking when he began to discuss fire stream flows in terms of gallons per second versus gallons per minute.

Have you ever heard this before? “If you take the XXXX fire formula, a typical bedroom fire only takes 40 gallons to control. It is over kill to take a 150 GPM fire stream to such a minor fire.” Hearing this thought process has always frustrated me but to be honest I always struggled with articulating a counter point until I began to consider the importance of exponential fire growth and gallons per second.

Let us say that a fire does double in size every 30 seconds. If the current bedroom fire takes 40 gallons to control and it doubles in 30 seconds, then 30 seconds from now it requires 80 gallons, and at 1 minute it requires 160 gallons. A 150 GPM stream is a 2.5 gallon per second stream. At 2.5 gallons per second, 40 gallons of water is delivered to that bedroom in just 16 seconds of operation. At 30 seconds of operation 75 gallons of water would be delivered to that room likely resulting in total room cooling not just fire control.

Chief Isakson’s message to shift the language fire streams to gallons per second could not be more appropriate. If the statement that a fire doubles in size every 30 seconds is wrong, so be it. You cannot argue that fire behavior in enclosed structures is changing faster than ever before and our windows of opportunity to act which were once measured in minutes have been reduced to seconds.

So if we are dealing with exponential fire growth, limited staffing and rapidly changing fire conditions the entire fire service should be evaluating their fire stream systems from the source to the nozzle not just a few inspired firefighters because we need to find ways to leverage our efforts at every point.

The Exponential Engine (FE Blog Post) Attack Lines.jpg

To provide a very brief overview before I expand on the idea, every engine company should be designed with a first due “Plan A” to attack whatever you have with all you have. Setting up a rig for with a plan for extended operations or waiting for the cavalry to arrive before you act only puts you closer to engaging a different fire than the one you are currently seeing (catch up). Variables will forever exist and nothing is set in stone but as firefighters we plan for a fight.

1 ¾”

The Exponential Engine (FE Blog Post) Flowing 45mm.jpg

One of the biggest pushes out there is greater volume from initial lines. Many fire departments are choosing 15/16” smooth bores or 185 GPM fogs, some even experimenting with 2” hose and 1” tips for the foundation of their fire attacks. If training, district construction and staffing make these viable options great; you are taking big weapons to the fight early on.

In my experience the initial handline for residential fires (a room or rooms on fire) for most fire departments is the 1 ¾”.

The benefits of the 1 ¾” attack line is that it supports a good fire flow for these size fires and it is very maneuverable for working on the interior of smaller compartmentalized occupancies. I think it is important to play up these strengths of the 1 ¾” and be cautious of the diminishing returns that the 3 person engine company encounters when too much is asked of this line. If our minimum interior attack fire flow is 150 GPM then the key operational range for the 1 ¾” attack line is between 150 and 185 GPM. Working above this range in volume starts to creep into high friction loss ranges and nozzle reaction forces especially if nozzles are used with operating pressures greater than 50psi.

What is key to remember is that nozzle ratings are just “ratings”, when closed all nozzles flow 0 GPM. A 150 GPM or 2.5 gallon per second nozzle may seem “inferior” to one that flows 185 GPM or 3 gallons per second, but if the nozzle firefighter can comfortably flow that nozzle for 30 seconds at a time around a corner while actively playing it they are delivering 75 gallons to the fire environment. A nozzle firefighter that is struggling with a 185 GPM or 200 GPM nozzles that can only operate it for 10 to 15 seconds at a time without fatiguing, and has poor stream movement is potentially ineffectively applying only 30 to 50 gallons to the fire environment at a time.

This is why I think the foundational line of the exponential engine company should be the 1 ¾” line so it can be rapidly deployed and easily maneuvered into position in the fire building with a nozzle flowing 2.5 gallons per second from a 150 GPM at 50 psi fog or 7/8” smooth bore with a nozzle reaction of 60 lbs force or less.

2 ½”

The Exponential Engine (FE Blog Post) Flowing 65mm.jpg

“I will not dispute that 2 1/2-inch hose is difficult to use, but no combination of smaller hand lines can duplicate the volume, reach, and pure knockdown power of a single, well-placed 2 1/2-inch line.” -- Andy Fredericks

At this point I don’t see the need to review ADULTS or get really detailed into when we should pull the 2 ½”. I think if you have hung on this long in the article you can recognize a fire that demands the 2 ½”. The struggle seems to come when the discussion shifts from when to how, especially with the 3 person engine. The most common concerns are that it is such a bigger and heavier line; in these concerns about the use are the keys to its use but we need to have more realistic expectations.

The 2 ½” attack line is not an 1 ¾”; accept that and move on. The 1 ¾” is the lightweight fighter; it can skip around the ring quickly for all 12 rounds and with great agility. The 2 ½” is the heavy weight fighter, it will move slower but with purpose, there can’t be wasted energy, and it is hoping for a early knock out so it doesn’t have to go the distance. In short the 2 ½” can be used very effectively with limited staffing it will just be a little slower and not nearly as long of an engagement but the punch it delivers is a big enough benefit that it is worth it. Attack whatever you have with what you have and understand the purpose of gallons per second.

The Exponential Engine (FE Blog Post) 65mm Knockdown.jpg

The series of pictures above is a single firefighter putting a 2 ½” attack line in to service on a working fire while his officer sets up the line for advancement after the knock down. The nozzle is an 1 1/8” smooth bore and the line operated for about 30 seconds from the parking lot before it was shut down and advanced into the stairwell for follow up. In that 30 seconds 132 gallons was delivered and it made a significant difference on that fire. While a second alarm was instantly called for on arrival of this fire, the quick action and rapid delivery of water prevented this fire from growing to the point where those resources were needed.

While working with another firefighter recently we reviewed this video and his comment was “That is a great example of using what you have. Too often we drive around in fire engines and act is if we don’t have tanks of water” His point, my point and Chief Isakson’s point is that if you view the 2 ½” as a 250 gallon per minute line then your thinking will inherently fall to flowing for minutes, and you will talk yourself out of using it because you believe you do not have the ability to support it. If you look above at the effect that 132 gallons delivered over 30 seconds had on that fire and you imagine your 500 gallon tank allowing for that kind of knock down to be followed up almost 3 more times you should see that you are not giving yourself near enough credit for your capabilities and you are just sitting on your opportunity to make a difference.

The above attack used an 1 1/8” smooth bore which flows 265 GPM or 4.4 gallons per second which is impressive but the idea of an exponential engine is exponential increases and we started with a 150 and 161 GPM line so the goal would be to put at least a 300 GPM 2 ½” into service as our next option delivering 5 gallons per second.

In order to place a 300 GPM attack line into service nozzle selection is very limited. A 100psi fog nozzle delivering 300 GPM would have a nozzle reaction of 150lbs force and would be extremely difficult for any firefighter to utilize in anything other than a fully defensive position. At 50 psi the 1 3/16” smooth bore delivers 296 GPM or 4.9 gallons per second with 111lbs of nozzle reaction and the 1 ¼ smooth bore delivers 328 GPM or 5.5 gallons per second with 123lbs of nozzle reaction.

Of these tip choices I personally would feel comfortable with either as a weapon just as I would with the 150 GPM at 50psi fog or the 7/8” smooth bore on the 1 ¾” attack line. There is one thing I did find particularly interesting about the 1 3/16” tip when you apply our true 2 ½” friction loss coefficient compared to the 1 1/8” tip with the traditional IFSTA based coefficient.

  • Friction loss per 100’ of 2 ½” hose flowing 265 GPM from a 1 1/8” smooth bore tip using the IFSTA coefficient of 2 is 14 psi
  • Friction loss per 100’ of 2 ½” hose flowing 296 GPM from a 1 3/16” smooth bore tip using a modern true hose internal diameter coefficient of 1.4 is 12 psi

Technically if you are using IFSTA based coefficients and modern 2 ½” hose you could just go replace all the 1 1/8” tips on your attack lines with a 1 3/16” and you would be flowing over 300 GPM or 5 gallons per second from your lines without anyone even knowing. Furthermore the very low operating pressure you may find the opportunity to eliminate a step for your pump operator.

It only requires a pump discharge pressure of 68 psi to flow a 150’ attack line with an 1 3/16” tip flowing right at 5 gallons per second. I have seen this operating pressure to be nearly idle for many modern fire pumps. Imagine if you set up a preconnected 5 gallon per second attack line to the point that it could be supported at idle. The speed which big water could be applied if all your pump operator would have to do when they got out of the cab would be to pull the tank to pump and discharge, eliminating the need to throttle up.

I know we clearly outlined working limits for nozzle reaction and the fact that this plan is intended for the 3 person engine company but if you remember the challenges of a bigger heavier line are also the keys to its use.

When you are advancing or dragging a line, friction is your enemy because you want the line to move forward into position with as little resistance and work as possible. When you are flowing a line friction becomes you friend because you can use it to absorb and counter nozzle reaction. The bigger and heavier a line the more friction is present and well trained operators can capitalize on that friction to serve as a back-up man in absorbing and grounding nozzle reaction. Additionally the larger diameter hose creates a more solid pipe and allows for more line to be moved ahead of the body resulting in greater stream movement without exaggerated body movement.

The Exponential Engine (FE Blog Post) Handling 65mm.png

In all these pictures the single operator is flowing between 265 and 300 GPM using line weight, the ground, a curb, wall or good body form to handle the higher nozzle reaction. As stated above this makes it a much less mobile line when compared to the 1 ¾” and it will most likely only be operated in a hit and move process but what is compromised in mobility is made up for in stream reach, punch and an extinguishing power that has been doubled without any staffing changes.

The Deck Gun

The Exponential Engine (FE Blog Post) Deck Gun.jpg

The deck gun might be a regional term, I have also heard it called the monitor or the “Stang” but we are talking about the engine mounted master stream. Most engine mounted master streams fog, or a stack of smooth bore tips have a flow range of 500 to 1000 GPM.

If we review our progression, the 1 ¾” flowing 2.5 gallons per second is a rapidly deployed and highly mobile attack line for a room or rooms of fire. The 2 ½” attack line flowing 5 gallons per second is our heavy weight fighter looking for the big knockdown against the big opponent of a full residential floor on fire, commercial occupancy fire or any of the ADULTS situations. Finally we have the deck gun for those marginal situations where you arrive to find an entire building on fire and rapid application of your entire tank at 10 gallons per second is required to nuke the fire’s progress and prevent extension to exposure occupancies.

2.5 gallons per second doubled is 5 gallons per second. 5 gallons per second doubled is 10 gallons per second so our target rating for a deck gun would be 600 GPM. The point of picking 600 GPM as the target flow for the deck gun goes beyond just the goal to double the volume of our previous attack level. Engine mounted master streams outfitted with a series of stacked smooth bore tips are most commonly found with an 1 3/8”, 1 ½”, 1 ¾” and a 2” tip.

Smooth Bore Tip Sizes and Stream Volume at 80psi

  • 1 3/8” 500 GPM
  • 1 ½” 600 GPM
  • 1 ¾” 800 GPM
  • 2” 1000 GPM

When a smooth bore of these diameters are used as master streams the operating pressure is 80psi and because the apparatus is the platform of operation nozzle reaction is not needed to be considered. Most of the times I check engines with these stacked tips I find that the full stack is in place with the 1 3/8” on top. Two reasons for this are because they came that way, or because the engine has a 500 gallon tank and to use a 500 GPM tip would give nearly a minute of operation before a supply is needed.

Once again this is thinking in minutes and trying to make what you have last over making what you have matter. I recommend the 1 ½” tip as the first up on the deck gun. Having tried this on a variety of different engines I have found this 10 gallon per second stream to be the highest volume, best quality stream that can be delivered strictly from tank supply. As you begin to move to the 1 ¾” and 2” tips a lot is being asked of the unsupported pump and internal plumbing and the lager tip size reduces the stream reach and pin point accuracy that you find in the 1 ½” tip.

Exponential Engine Summarized

There are hundreds of potential options and combinations for means of fire attack at the disposal of today’s firefighters; this approach is just one of them. My belief is that while there are hundreds of options, the three person engine can really only perform one action at a time and this is one way to attempt to simplify and maximize the effectiveness of these first due actions.