Do you absolutely, positively need an electronic engine monitor? Or are they really just cleverly packaged microprocessors competing for the bucks youd otherwise spend on a new navcomm?
Consider the science and physics of airplane engines. Theyre nothing but heat engines, converting thermal energy to useful power. The temperature of various parts of the engine, therefore, is a useful indicator of the engines health, power output and efficiency. In short, engine monitors offer an inside look at engine operation that the standard cockpit instruments cant touch.
The market is flush with choices. Besides price, a key differentiation between the various models of monitors is the number of different temperatures being measured. First-generation/inexpensive monitors typically sample only one temperature, such as single-cylinder CHT. The latest generation monitors are multi-probe units and can sample, integrate and display temperatures from multiple probes. Moreover, they can electronically log and store temps for later analysis.
Cylinder head and exhaust gas temps are the two critical parameters here. EGTs are a marker for combustion efficiency and ignition system health. The actual absolute EGT value is less important than comparing it to trends and to CHTs.
CHT tells you about engine cooling, or the lack thereof. Unlike exhaust gas temperature, actual numbers for CHTs do matter. If the CHT is too low, the engine will not have achieved its proper steady-state material dimensions. If too high, detonation margins diminish and, worst case, cylinders can actually deform.
With optional inputs, engine monitors can also sample turbine inlet temperature (TIT) on turbocharged engines, oil temperature, carburetor (induction) temperature, and outside air temperature (OAT). Some models also have fuel flow functions, voltage monitors and programmable alarms.
Bars and Numbers
Religious arguments about information presentation run roughshod through the engine monitor industry. For a time, the prevailing wisdom was that pilots were best served by an analog or graphic bar display of all cylinder temperatures simultaneously-a philosophy followed by JP Instruments and Insight.
If we can ignore the questionable label-theres nothing analog about a 15-segment bar graph with discreet 25-degree increments-the idea was to apply scientific visualization to the stodgy field of instrumentation. In other words, draw some pretty pictures from the raw numbers and convert the display from a presentation only a sports statistician could love into something more akin to the sound board at a Stones concert.
However, in the everything-old-is-new-again department, two manufacturers now tout the fact that their units lack the bar graphs and, instead, display the raw numbers, attracting the pilots attention only when a critical parameter has been exceeded and, perhaps, as a courtesy, when his Gold Medallion warranty has expired.
In addition to basic data display, all of the units we examined feature some type of temperature alarms (except for the EGT-only Tetra I and Hexad I). The units differ only in how many types of alarms they have with the Insight and KSA having the fewest. The JPI, Allegro and EI all sport more programmable alarms than Microsoft has pending lawsuits.
The Insight, JPI and Allegro all feature a leaning aid. This works by monitoring each cylinders EGT and giving an indication when the EGT trend, for any cylinder, reaches peak or leanest operation.
Opinions were mixed on the utility of this function. First, it requires a certain amount of operator skill; lean too slowly and the units can be fooled by false blips from the temperature probes (especially those with cheap probes). Lean too fast and the units cant keep up.
Also, with the exception of the Allegro, using the peak find option requires prodigious eyes-in-cockpit time. Most pilots that we spoke to who had the feature admitted that they rarely used it.
Display normalization is available on all the graphic-type monitors, including KSA, JPI and Insight. This refers to the ability to visually align the display bars across the face of the instrument. Normally, no engine will have uniform EGTs, yielding a jumbled, sawtooth display. By normalizing or aligning the bars, a trend or early onset of problems is supposedly easier to detect.
With these basic considerations in mind, lets review the offerings.
KS Avionics (KSA)
KS Avionics makes two models of their analog multi-probe engine monitor. The Tetra I and Hexad I, differ only in the number of cylinders monitored, four and six respectively. The standard version of each features simultaneous EGT monitoring of all cylinders only. A II version of each adds CHT monitoring (although not simultaneous with EGT; you must switch between the two modes with a toggle) as well as alarms for high CHT and excessive CHT cooling rate.
Excessive CHT cooling is preset at the factory as anything more than 40 degrees F per minute per cylinder. Both cooling rate and high CHT temperature alarm points can be changed in the field, however.
The Tetra and Hexad units are classic true analog electromechanical-style devices. Theres no digital readout of absolute temperatures nor are the pointers rendered with light emitting diodes. Instead, the pointers are driven electromechanically via a separate amplifier module.
The module also contains the controls for setting the CHT alarm and shock cooling limits for the II versions. The units also feature knobs below each cylinders indicator which can be used to normalize the display.
KS Avionics supplies a nice typewritten practical guide detailing the intricacies of engine temperature management with their units as well as background information on detonation, pre-ignition and the usual list of abnormalities that an engine analyzer will detect.
While KSA units may appear dated-the design originated in the 1960s-the company and the product has a solid, some would say fanatical, reputation for quality. Some pilots, frankly, simply prefer the analog steam gauge format over the diode-type display; especially when the other instrumentation in the aircraft is of the same type.
EIs top-of-the-line entry in the engine monitor arena is the Ultimate Engine Analyzer. It has 16 channels divided into eight groups of two channels each. Each group can display both of its associated values simultaneously via two numeric LCD displays. Thus, a typical set-up might have group one showing the simultaneous EGT and CHT values for cylinder one, group two the same for cylinder two, etc. Its important to note that because the units primary feedback mechanism is two LCD displays, the values of only one group can be viewed at a time. In normal operation, the unit will step through the groups, sequentially displaying all measured temperatures.
Since there are eight groups, that means that on a typical four-cylinder engine, only half the groups will be used if the engine is instrumented for both CHT and EGT on all cylinders. On a six-cylinder engine, two thirds of the groups are used. Each still provides plenty of additional inputs which can be hooked to oil temperature, TIT, outside air, carburetor air, etc.
Each channel can have an associated high and low temperature limit alarm. The two LCD displays can also have both differential and trend limits assigned to each of them.
Differential limits are useful for spotting individual cylinder anomalies, such as a cylinder with an abnormally high or low CHT with respect to the others. Trend limits are most useful for spotting shock cooling problems. All told, the Ultimate Scanner can provide a visual alarm for 36 different temperature events.
The Ultimate Scanner features a peak temperature finder. Dont be misled; this is not the same as the lean find mode found on other monitors. The Ultimate Scanners peak find spots only the hottest absolute temperature. It does not automatically determine when a given channels temperature has peaked, a function critical to automatic determination of peak EGT.
EIs Ron Roberts told us the companys engine monitoring philosophy is that data should be in the cockpit, but that the pilot shouldnt be overwhelmed with raw data. Let the computer take care of that and only let the pilot know when something goes out of spec. Nevertheless, EI has recently added data logging to the Ultimate Scanner and they plan on announcing the capability at this years Sun n Fun Fly-In. The Ultimate Scanner will output the raw data-unfiltered and uncompressed-from each of its temp sensors into a portable computer.
Electronics International has a strong customer following and the quality of their products is well regarded. From personal experience, we can say the units are well made and robust, especially the temp probes.
Allegro Avionics model M816 Lynx is quickly becoming a darling of the homebuilt crowd. However, you may run into a snag installing it in a spam can, as it has neither PMA nor STC approval. Given a willing mechanic, a complete Form 337 and a co-operative FSDO, anything is possible. However, Peter du Bois of Allegro explains that the company is not encouraging installs in certified airplanes.
In a gutsy marketing move, Allegros glossy collaterals denigrate fancy electronic bar graph displays. Were not sure that we agree but we admire contrarians nonetheless.
Allegro argues that the majority of engine problems identify themselves as trends, such as a CHT slowly trending up or an EGT trending down and that trends are difficult to spot on a bar graph. The bars, says du Bois, are good at identifying a single catastrophic failure; a dead cylinder. However, failures are rare and usually identifiable by other means. (Say, did you catch if the cylinder that just departed had number 5 or number 3 stamped on it?)
Allegros design concentrates on two criteria with regard to engine health and efficiency: the maximum temperatures of any cylinder and the maximum span in temperature between a given cylinder and the rest. With a bar-graph type display, its more difficult to easily discern which cylinders have the highest EGT, CHT or span of the two values.
This view has merit. For example, an Insight GEM with 25-degree bars could visually indicate that all cylinders are equal of temperature when, in truth, theres a substantial variation between the hottest and coldest probe. On the Allegro, the comparison is done in the unit, requiring less eyes down in the cockpit.
Allegros unit has a true lean-find mode. Additionally, it has an audible tone to indicate when the first cylinder has peaked. This is an improvement over the other lean-finding analyzers which require periodic visual scanning. Allegros unit also has a user programmable shock cooling alarm that can give a pilot an indication that cylinder cooling is occurring beyond a set value.
The four-cylinder Lynx comes complete with eight temperature sensors (one each for each cylinders CHT and EGT), provisions for a pressure port for oil/fuel/manifold pressure and sensors for bus voltage and bus amperage sensors.
The six-cylinder Lynx has additional temperature sensors plus provisions for another pressure sensor. Both models can be upgraded with up to a total of 16 temp sensors for TIT, oil temp, outside air temp and so on. Both will also accept an optional fuel totalizer option and a GPS interface option. The GPS option, when used with the fuel totalizer, will display gallons to destination as well as alarm should you have less fuel than you need to make it.
Insight Instruments virtually invented the graphical analog engine monitor as a marketing concept. While other multi-probe instruments preceded them (such as the KSA and Alcor), Insight was the first to combine modern digital electronics and luminescent displays into a single package.
Insights flagship model 602 took the market by storm in 1982, with its simultaneous electronic display of all EGT and CHT engine information. The 603 adds TIT input for turbocharged engines.Both models are still available and are still excellent basic analyzers.
Insights current top-of-the-line model is the model 610, introduced in 1993. It improves on the original by adding digital numeric temperature outputs to the bars, trend indicators, a TIT input, OAT and data logging capability.
Insight also should be credited with producing some of the most informative pilot materials available, including a videotape, about engine temperature and off-line trend monitoring.
However, Insights designs are beginning to fall behind other offerings in terms of options; Insight isnt quite keeping up in the race to take home the creeping featurism trophy. Most notably lacking in the 610 are programmable temperature alarms for each channel, in-flight trend and difference monitors, electrical monitors, fuel flow options and everything else designed to differentiate oneself from just another integrated temperature sensor.
On the other hand, set-up of the model 610 is probably easier than any other model on the market. Instead of a complex programming process involving pushing several small buttons, the 610 can be configured with an HP 100LX handheld computer. This is included with the unit or, if you have your own, theyll knock $500 off the price. It uses a menu system and talks to the monitor through a wireless built-in infrared port.
The 610 is also the only unit, currently, which comes standard with onboard data storage. In flight, the unit constantly records temperature parameters and stores them with time stamps in its non-volatile memory. Theres no need, as there is with the JPI, for a separate data storage device.
Insights Bill Freeman counters the claims of manufacturers such as Allegro that trends are difficult to discern with the GEM. He argues that alarm-type units have to have wide margins to avoid constant nuisance alerts and wide variations will show on the graphical display. Furthermore, says Freeman, alarm-based displays mask subtle trends which the GEM-type display will show.
The option of 20 to 30 hours worth of data storage on the GEM 610 is significant for the owner-operator who doesnt see himself crunching numbers on every fight but does like the ability have a record of the engines last few hours of operation should something abnormal pop up. Savvy mechanics like this feature, too.
The downside of the 610s built-in data logging is that Insight has chosen to compress the data in order to increase the amount (in hours) of data that can be stored. Apparently this compression is lossy and results in a slight but arguably important loss of precision.
JPIs EDM-700 has come a long way since Aviation Consumer did its first review of the unit in 1993. Gone is the poorly written laser printed instruction manual, the thin grounded EGT probes and the stunningly expensive data-logging option. Now, theres a glossy pilots operating manual, robust grounded probes, fuel flow measurement and standard data logging.
The JPI has virtually all the bells and whistles found in the other units reviewed, with the exception of data storage. For the JPI that must be provided by an external unit such as a PC or JPIs expensive data recording unit, either of which must be plugged into the analyzers serial output. The JPIs data presentation includes both digital and bar-graph displays.
JPIs unit incorporates an automatic scan and display of all measured parameters, thus you can set it up to momentarily scroll through CHTs, EGTs and oil temps. The GEM 610 cant do this. Other significant features of the JPI that are missing in the 610 include alphanumeric indications of whats being displayed (OIL), a bar graph for TIT and wider PC compatibility for logging.
We heard some grumbling about the user interface for the JPI. In particular, pilots dont seem to like the single button interface. They would prefer a multi-button design that allows some degree of backward and forward scrolling. The semi-intuitiveness of the interface was demonstrated on one flight where we were constantly fetching the owners manual to find how to switch to a different mode. Operation is learnable, of course, but not what we would call streamlined.
Flying with JPI owner Robert Kail, we noticed a downside to all this automation: He fixated on the shock cooling alarm and admitted that he had changed his flying habits in order to avoid tripping the dreaded flashing warning. While weve paid our share of engine overhaul costs and can understand the raw fiscal emotion the shock cooling alarm elicits, we think this feature probably causes more agita than its worth. Our view is that in most engines, shock cooling is an overrated myth.
Its impossible to say which of these units is the best since they differ significantly in design philosophy. If youre looking for a dirt-simple and reliable semi-mechanical unit then the KSA Tetra/Hexad units are a good choice. Theyve been around forever and have a solid reputation.
For a basic bar-graph unit with lean find but no datalogging, the GEM 602 is as good as it ever was and is still available at a bargain price. If bar graphs bore you and you think you can slide by with a field approval-or you have an experimental-we like the Allegro over the EI because of the additional inputs, greater flexibility and lower cost of the Allegro.
Now, on to the big two: Insight versus JPI: In our view, for bar-graph style units, the JPI EDM-700 has the edge over the GEM 610 in terms of features and functionality. With the exception of a built-in data storage capability, the JPI has every feature of the GEM plus several (such as alarms, more inputs, and a cleaner data logging capability) that the GEM lacks.
Note, however, that the JPI unit is reported by some dealers as having a higher infant mortality rate than the GEM. JPI has attempted to improve their quality-especially probe quality-and that seems to have had a positive effect. The jury is still out, however, on whether theyve caught up to Insight in terms of overall quality control.
All of the companies we dealt with were responsive to inquiries regarding their products. Especially impressive was JPI who FedExd their information (without being asked to do so) and Insight who wins the award, in a tight race with EI, for attempted murder via deluge of glossy brochures. Similarly, weve heard no complaints about any of these companies with regard to support and repairs after the sale.
Also With This Article
Click here to view the Engine Monitor Checklist.
Click here to view the Engine Monitor Comparison.
Click here to view “Installation Notes.”
Click here to view the Engine Monitor Addresses & Contacts.
-by Gregory Travis
Gregory Travis is a Cessna 172 owner and writer. His Web site contains in-depth information on Lycoming engines. (www.prime-mover.org/Engines/)