Brian Callahan

ME 590

for Prof. Steven Ceccio

and Dr. Stuart Cohen

University of Michigan, Ann Arbor


An Inertial Dynamometer and Radio Telemetry System for High Speed, Glow Ignition, Two-Stroke Racing Engines


An inertial dynamometer was built to measure torque and power output from small, high speed, two stroke, glow ignition, racing model marine engines. A discussion of system construction is presented, along with sample measurements. A radio telemetry system was built to measure engine speed during actual on-water operation, for comparison to bench measured power characteristics. Sample telemetry results are also presented.


Nitromethane / methanol fueled, two stroke, glow ignition engines power radio controlled high speed model race boats.

Figure 1: A Roadrunner 11cc Hydroplane turning at 6.7g's

Previously, little was known about the engine speed behavior during racing situations. Overall hull lengths shorter than 1m, weights less than 2.5 kg, sustained accelerations greater than 6 g's, a corrosive marine environment, intense vibration, and unknown propeller characterizations have prevented even the simplest data acquisition. In addition, engine speeds in excess of 32,000 revolutions per minute, sensitivity to small torque loads, and unavailability of gear reduction units have prevented thorough characterization of engine performance.

This project intended to solve these problems with the introduction of a small, high speed inertial dynamometer and a small, lightweight radio telemetry system. The dynamometer was designed to measure torque at engine speeds and velocity transients similar to actual operation. The telemetry system was designed to transmit engine performance data from an operating vehicle to a data acquisition system on shore with minimal impact on hull dynamics.

Inertial Dynamometer

It is possible to calculate engine torque and power by allowing it to accelerate a known inertial load, measuring instantaneous acceleration, and mapping the results back onto the corresponding engine speed [1]. The dynamometer built fit onto an aluminum frame 1m X 1m. The frame provided a structure for attaching the engine, the inertia wheel support bearings, a safety scattershield in case of wheel rupture or disconnection, support for the driveshaft, a fuel tank mount, and engine water cooling lines.

Figure 2: The complete dynamometer

A steel disk provided inertial load, a wound cable delivered engine power from the crankshaft to the load, a one-way clutch pressed into the load prevented cable damage and allowed engine coast-down, a servo controlled the throttle and provided means for a rev limiter, and a computer data acquisition hardware and software system captured the data, reported results, controlled the servo during rev limiting operation, and provided a user interface.

The dynamometer was designed to handle engines displacing 3.5, 7.5, 11, and 15cc with shaft speeds up to 33,000 revolutions per minute on the 3.5's, and power levels up to 5.2 kW on the 15cc engines. The inertia wheels were designed to simulate engine accelerations ranging from 5,000 to 10,000 rpm/sec.

Inertia Disk Design

To accurately simulate velocity transients similar to actual racing conditions, several inertia disks were designed to match a variety of engine and hull combinations (See Figure 3).

Figure 3: Inertia disks

The choice of a thick cylinder provided a geometry convenient for calculating rotational inertia, easy machining, and minimum air drag. Readily available 4140 steel served as a suitable material due to its machineability, high density, and high tensile strength. Preventing burst failure dictated a diameter that kept tip velocity lower than 150 m/s at the particular engine's maximum speed.

Wound Cable Drive Shaft and One-Way Clutch

To accommodate steady state loading necessary for engine tuning and break-in, the inertia wheel was replaced by a choice of large diameter, accurately machined fan blades, each accurately machined to provide a suitable load for the particular engine, speed, and load condition. To package these blades, the dynamometer output shaft needed to extend off the end of the bench, and perpendicular to the engine mount (see Figure 2). A wound cable housed in a brass tube allowed engine power to travel through the 90 degree turn. This also had the beneficial side effect of accurately simulating drivetrain losses in the boats, since they use identical components.

Unfortunately, a wound cable is capable of resisting torque in one direction only, while the inertia wheel resists both acceleration and deceleration. Any time the inertia wheel overran the engine, even for a brief moment, the cable failed (see Figure 4).

Figure 4: Cable failure due to reverse torque

A one-way roller clutch pressed into each inertia wheel allowed overrun without imparting any reverse torque. The clutch also allowed the engine to coast back to idle after each test, which helped prevent scuffing damage in case of a fuel lean condition during closed throttle operation. During initial testing, the clutch rollers tended to brinnell the output shaft, which lead to catastrophic clutch slippage in a matter of two runs. Precisely machining the output shaft diameter to the clutch manufacturer's specifications, switching to 8620 alloy steel, and case hardening to a minimum of 58 Rockwell C prevented any more impact damage.

Rev Limiter and Throttle Control

Flame ignition in these engine is accomplished by a passive glow plug element. Since there is no control mechanism to turn off the ignition, the engine will continue to accelerate unless the intake or exhaust is throttled, the fuel flow stops, or the resistive load increases to match the excess torque. If left unloaded, some of the engines will reach a speed high enough for mechanical failure.

Since the inertia wheel and bearings were designed to have a minimum air drag and friction resistance, they can supply little resistive torque at small accelerations.

Resistive torque,

Also, the resistive torque is independent of rotational speed. Therefore, the inertia wheel will not stop the engine from climbing to mechanical failure.

To prevent engine damage from excessive speed, an electronic throttle control was built. A standard, high-speed servo similar to the boats' on-board throttle control governs the throttle setting (see Figure5).

Figure 5: Servo control mounted to the engine throttle

A solid state circuit took as input a potentiometer voltage from a hand control (see Figure 6), and a digital input from the computer data acquisition board. Under normal operation, the manual control operates the servo. But, if the monitoring software in the computer notices that the engine is above a prescribed limit, it signals the digital input, and a comparator circuit commands the servo to an idle position.

Figure 6: Manual throttle control potentiometer

Data Acquisition Hardware and Control Software

An aluminum disk swinging through an optical coupler to generated a square wave signal with frequency proportional to engine speed. A National Instruments 1200AI board with the 5B37 thermocouple module and frequency-to-voltage converter installed in an IBM style Pentium 90 computer provided a robust data acquisition system.

The software took as input only the engine speed voltage. It calculated instantaneous acceleration at each time step, and monitored the rev limit control. The user interface allowed the operator to choose the specific inertia value for the chosen wheel, prescribe a rev limit, input the engine bore and stroke, manipulate the output graphs for convenient examination, and record test notes. It reported torque and power at every rpm, magnitude and position of the torque and power peaks, and maximum Brake Mean Effective Pressure.

Since calculating acceleration involved taking the first derivative of the speed signal, the calculations were highly sensitive to noise [2]. Initial testing produced results with signal-to-noise ratio of almost 1:1. Switching from software timed to hardware timed acquisition, reserving all calculations for post processing, and adding a software simulated low-pass filter with carefully chosen coefficients allowed noise free, yet highly accurate results.

Test Method

  • Start and warm the engine.
  • Throttle the engine to a comfortable idle.
  • Slow the inertia wheel to match engine speed.
  • Begin data acquisition.
  • Bring the manual control to wide open position.
  • The engine accelerates at maximum power, until the software notices max rpm, signals the servo controller to bring the engine to idle, ends data acquisition, begins calculations, and reports results.

    Meanwhile, the engine slows to idle, and the inertia wheel overruns on its one-way clutch.

    Once the software has reported results (less than 100msec), it continues controlling engine speed in a closed loop with hysteresis.

    • For the next test, hold a steady idle, brake the wheel, and start again.

    In this fashion, the dynamometer operators can complete several tests per minute, with the complete performance characteristic displayed real-time.

    Sample Dynamometer Results

    The Results in Figure 7 show a sample dynamometer run on an 11cc Picco EXR engine intended for a lightweight hydroplane hull. The inertia wheel chosen approximated 8000 rpm/sec engine rise. This engine developed excellent torque and power.

    Figure 7: Sample dynamometer user interface, with sample test results

    Radio Telemetry System

    The same trigger wheel and optical coupler from the dynamometer served as the engine speed detector. The signal was transmitted to shore using a battery powered Frequency Modulated transmitter. On shore, a radio receiver demodulated the signal, and the same frequency-to-voltage converter from the dynamometer sampled the data.

    The entire on-board system weighed less than 75g, which is suitable for even the lightest of hulls.

    Figure 8: The complete on-board radio telemetry system

    Sample Telemetry Results

    Shown in Table 1 are results from an on-water test with the same engine that produced the results in Figure 7, in the 3.5 kg hull pictured in Figure 1.

    Table 1: Sample Telemetry Results


    Once discovered, the complete power versus engine speed behavior, coupled with the on-water engine speed measurements, will quickly enhance engine performance development. The tools built during this project will also be useful for vehicle dynamics improvement, engine durability development, and driving method enhancements.

    An entire test takes less than five seconds to complete. Compared to other test stands which employ steady state measurements, this method is much less taxing on the test engine. Since the stand is intended to test hundreds of engines each year, this can represent much saved cost in otherwise destroyed engines.

    Recommendations for Future Development

    Increasing the radio telemetry's range to fully encompass the standard 1/6 mile racing course will allow much information about hull and engine dynamics. Especially important is measuring actual engine acceleration, so that the dynamometer's inertia wheel can be precisely matched to each hull and engine combination.

    Continued engine testing will highlight specific characteristics of the dynamometer operation. For example, sensitivity to engine temperature during the test can be an important factor [1].

    Measuring exhaust gas temperatures during the runs is critical for exhaust chamber development. A thin, low time constant thermocouple needs to be employed to capture the temperature during the short run.

    Adding an exhaust pressure transducer could also aid exhaust chamber and exhaust port design. Unfortunately, the necessary sampling rates in excess of 50 kHz will make this challenging.


    The author wishes to acknowledge the contributions of each of the MWD and Associates team: Marten Davis for his hospitality, financial contribution and use of his engines, John Ackerman for his engine expertise and use of his shop facility, Norris Sparks for his machining labor and expertise and use of his engines also, Joe Kramer for his power supplies, and Bobby Coleman for all those barbecued ribs.


    [1] Kee, Robert J., Blair, Gordon P., Acceleration Test Method for a High Performance Two-Stroke Racing Engine, SAE Motor Sports Engineering Conference Proceedings Dearborn, Michigan, December 5-8, 1994. SAE paper 942478

    [2] Tang, Liang, Signal Processing Question. Personal interview (21 June 1997).

    ENGINE Analysis Software for the Serious RC Competitor



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