Governors versus Relief Valves: Why the Conflict?

November 5th, 2007

by Doug Kelley

One of the more vocal technological debates in the fire apparatus world concerns the system used to control the pump. In this world, there are two main systems. The first system consists of a direct engine speed control combined with some type of mechanical pressure relief valve. The second system consists of a digital pressure monitoring system (i.e. a governor) that automatically adjusts engine speed based upon the operator’s input.

These two systems are not without their controversy. Some of the most common questions that any fire truck manufacturer receives concern why a department should choose one system over the other. Some of the questions arise because many fire officers grew up with mechanical systems and have a good deal of trust in their operation. These officers frequently request vernier throttles on their trucks and would prefer to stay away from the digital governor system. However, sometimes this is done reflexively without even understanding exactly how each system operates and the advantages and disadvantages of each.

By way of background, it is important to understand that there is no such thing as a mechanical engine control any longer. In the decades between the introduction of the automobile and the 1970s, some extremely clever mechanical control devices were created to control engine speed. Most commonly this consisted of a cable that attached directly to the throttle input on the engine itself. Such systems were easy to understand, and when something went wrong, they were easy to troubleshoot and repair.

However, these systems had two major disadvantages. First, they could not make the fine adjustments in fuel and air balance required to minimize pollution out the tailpipe. Second, since throttle position was really the only parameter the operator could adjust quickly, the engine was susceptible to running differently in different conditions (such as temperature). As a result, electronic systems were created in which throttle position was just one input in controlling how the engine operated. The throttle position was changed into a digital signal which could be combined with other digital inputs to properly and efficiently improve the engines’ performance. Manufacturers found that electronically controlled engines would run more consistently with less pollution. Primarily as a result of government regulation, starting in the mid-1990s, mechanically controlled engines had been phased out and were no longer available on new trucks.

This presented a challenge to fire truck builders. Fire truck component manufacturers had to figure out how to control the engine speed on an electronic engine for pump operations. The easiest approach was to model what existing fire trucks already had. At that time, almost all fire trucks were equipped with a direct throttle control paired with a discharge relief valve. Therefore, the logical approach was to simply replace the direct throttle control with the electronic throttle control. Manufacturers went so far as to duplicate the look of the old rotary throttle cable control so that the appearance of the pump panel did not change. However, the “guts” of that rotary control had been replaced with a variable resistor that gave a digital voltage signal directly to the engine controller. Overpressure and transient protection continued to be provided by the relief valve somewhere on the discharge side of the pump.

The system worked. It duplicated the same operating procedures that fire departments had been using for decades. However, though it carried forth the familiar operating characteristics, it also carried forth the same limitations. Specifically the system remained a reactive system fully dependent on the operator’s skill for consistent, safe operation. For example, if the engine load changed, such as by starting a generator or closing a discharge, the operator had to quickly readjust the throttle position to reestablish the correct pump pressure. In addition, the safety of the system (and the magnitude of the transients) was only limited by the relief valve setpoint. The relief valve could be adjusted within a certain range. However, the only way to set it was to run the pump at the desired discharge pressure, and then adjust the relief valve until the valve opened.

Since there was no way to tell at a glance where the setpoint was, the potential for large pressure spikes was very real.  For example, if a pumper was supplying multiple discharges, and one of them was shut at the nozzle, two things would happen. First, since the load on the engine decreased, the engine would race slightly and the natural engine pressure would increase. Second, the shutting of the nozzle would create a pressure surge through the entire system. The magnitude of these combined would only be limited by the proper use of the relief valve. If it was set correctly, fine. If not, then the firefighters on the ends of the remaining nozzles would have to hang on.

About the same time that the conversion to electronic engines was being completed, other designers were looking at the situation differently. Rather than duplicating what was already there, these designers saw an opportunity. It would be relatively easy to convert the pump pressure into another digital signal. A computer program could easily be written to measure the pump pressure against where the operator wanted it to be. That difference could then be used to change the engine speed until the two matched.  And thus, the first governors were created.

These governors were initially offered (and still are) with two operating modes. In RPM mode, the governor maintains a constant engine speed. This is most similar to the traditional throttle control with an important exception. It will maintain that speed regardless of engine load. In Pressure mode, the governor will adjust engine speed to maintain a pre-set pressure regardless of pump flow. In effect, the governor combines the function of the throttle with the relief valve and does what the operator used to have to do. Specifically, it monitors pressure trends and compensates to keep things smooth.

Therefore, the philosophy behind pump governor systems was very different from the throttle / relief valve system. Rather than being reactive, this system became proactive. By constantly monitoring the discharge pressure, the system could make changes much faster than any operator. Therefore, the engine speed could be almost instantly adjusted to smooth pressure transients. Some governors on the market will not allow more than 10 psi of spike under the worst conditions. In addition, the operating information could be displayed more readily. No more would the operator have to assume that the relief valve was set correctly. They could tell at a glance what the discharge pressure setpoint was.

The second and third generation governors were even more advanced. On every engine is what’s called a data bus. This is a data stream carried to any component that might need the information it contains. This includes such obvious parameters such as oil pressure, engine water temperature, and electrical voltage. But it can also include engine operating status, error messages, pump pressure, pump interlock status, and almost any other parameter the operator may find useful. Therefore, the latest governors are less about simply controlling the pump, and more about providing a centralized control center that can display the complete operation of the apparatus.

In short, governors are the state-of-the-art in fire truck operations. However, there are still those that are more comfortable with the old throttle / relief valve combination. Why? Well, for starters, there is comfort with the mechanical. Notwithstanding the fact that there is no such thing as a mechanically controlled engine any more, for the mechanically inclined, a data stream can be a scary thing. When a mechanical item breaks, it is often (though not always) an intuitively easier item to diagnose. However, if you are not used to electronic diagnosis, it may appear that something has broken with no visible cause. However, the truth is that the fire truck industry is one of the last industries to make the switch to digital control. The aviation and automotive industries made the leap decades ago. The fact is that a modern airliner would be impossible to control with mechanical controls and analog electrical systems. This is not new technology. Even in the fire industry, governors have been in general use for a solid decade now. The technology has been proven and can operate consistently and safely on the fire ground.

In addition, the nature of repair has changed. In the distant past, a fire house mechanic might be able to repair components. Now, regardless of whether the system is mechanical or electronic, the prevailing repair technique is isolate and replace. Therefore, whether the component fails due to a cracked casing or an electrical fault, the same techniques will be used. And skilled electronics technicians can fix a system just like skilled mechanics.

However, the most compelling case for governors is the changing role we are asking our apparatus to play. No longer is the engineer responsible for simply keeping water flowing through the hoses. The engineer has to do that plus keep area lighting sufficient plus monitor safe operation of A/C electrical systems plus monitor and relay fire ground communications plus monitor exterior conditions plus keep rescue tools operating correctly plus stay out of traffic. In this complex world, the more the apparatus can do for itself, the easier it will be for him to focus on the other things.

Of course, if a fire department prefers the throttle / relief valve system, they are still available. However, before making that decision, it would be wise to look at the governor systems available, and see how apparatus operation may be advanced over what it used to be.

This entry was posted on Monday, November 5th, 2007 at 12:00 am and is filed under commentary articles. There are One Response to “Governors versus Relief Valves: Why the Conflict?” :
Is a Single Pump Discharge Pressure Feasible? - The Rig | Firefighting Apparatus, Vehicles, & Equipment Says:

[…] governors and a comparison between them and pressure relief valves was done in 2007 by Dave Kelley ( I make no comment on the governor/relief valve discussion. The original doctrine of automatic […]

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