Forum: WHALER
  ContinuousWave
  Whaler
  Moderated Discussion Areas
  ContinuousWave: Whaler Performance
  Voice Coil Fuel Injector

Post New Topic  Post Reply
search | FAQ | profile | register | author help

Author Topic:   Voice Coil Fuel Injector
jimh posted 01-17-2013 02:03 PM ET (US)   Profile for jimh   Send Email to jimh  
Voice Coils

It is widely known that the patented E-TEC fuel injector uses a voice-coil design in the actuator, a significantly different approach than the usual solenoid coil. To briefly explain the difference, in a voice-coil actuator there are strong, heavy, permanent magnets that create a field. In this field a relatively light and nimble voice coil is moved by creating an opposing magnetic field with electrical current in the voice coil. The solenoid does the opposite. The coil is fixed and the permanent magnets move.

The voice-coil is well known to many by its use in loudspeaker design. I thought it might be interesting to compare the rate of oscillation in the E-TEC injector to that of a loudspeaker, as well as some other properties.

We know that in a loudspeaker the voice coil can typically be moved back and forth at audio rates, that is, from 40-Hz to 10-kHz. Voice coils in loudspeakers can be moved back and forth at even higher or lower frequencies than that, but for most cases of audio reproduction, 40-Hz to 10-kHz is a good range. This is a range of frequencies of 250:1.

An E-TEC fuel injector makes one cycle per cylinder combustion cycle, that is, the voice coil is moved forward and retracted once for each time the piston moves through one combustion cycle in the cylinder. In a two-cycle engine a combustion cycle is just one cycle of cylinder movement; the two are the same. If an E-TEC is idling at 500-revolutions per minute (RPM), each E-TEC injector is being operated 500-times per minute, or at a rate of 8.33-Hz. If the E-TEC engine is accelerated to 6,000-RPM, the E-TEC injector is operating at 100-Hz. This is a range of frequencies of only 12:1.

We see that a voice coil in a typical loudspeaker operates over a range of frequencies that is more than twenty times greater than the range of frequencies used in a typical E-TEC engine. This suggests that the design of the E-TEC voice coil could be optimized for a narrower range of operating frequency.

The voice-coil in a typical loudspeaker is generally only powered with about 5-Watts of electrical energy. If the loudspeaker is reasonably efficient, 5-Watts of electrical power will create a rather loud acoustic sound from the loudspeaker. The voice coil is continuously operated in the reproduction of audio. Now we compare to the E-TEC injector power.

The E-TEC injector is operated from 55-Volts. According to the service manual, the resistance of the typical E-TEC voice coil should be in the range of 2-Ohms to 3-Ohms. If 55-Volts is applied to 2-Ohms, we can anticipate a current to flow of roughly 20-Amperes. This implies a power of 55-Volts times 20-Amperes, or 1,100-Watts. This is likely the peak power input to the injector. The cycle time of the injector is very short. We know that injection of the fuel into the combustion chamber must be done only during a very short interval in the cylinder rotation cycle. The E-TEC injector only operates during this short interval. The rest of the time it is not being supplied with electrical power. We can roughly estimate that the time during which fuel must be introduced into the engine ought to be only during about 36-degrees of the cycle. This means the E-TEC injector duty cycle is thus roughly 36-degrees/360-degrees or about 0.1. The average power of the injector is therefore 1,100-watts x 0.1 or 110-watts. The time that the E-TEC injector is operated is in proportion to the engine speed. I have assumed that the 0.1 duty cycle is typical for the maximum engine speed of 6,000-RPM. At lower engine speeds the duty cycle is likely to be proportionally lower. At the idle speed of say 500-RPM, the duty cycle is likely to be only 1/12th, or about 0.0083. This would suggest an average power of about 10-Watts into the injector. Applying from 10-Watts to 110-Watts of electrical power to the injector will create mechanical movement, but some of the energy will become heat. The circulation of fuel through the injector body will tend to carry away the heat created.

Now we compare the total number of voice-coil cycles in loudspeakers and E-TEC injectors. Let us say the average frequency applied to a loudspeaker is 800-Hz. This means the voice coil averages 800-cycles per second. This is a rate of 48,000-cycles per minute, or 288,000 per hour, or 25,228,800,000-cycles-per-year. It is not unusual for a loudspeaker to last for ten years, even under continuous operation (say as a monitor loudspeaker in a broadcast control room). This implies a total number of cycles of the voice coil of 252,288,000,000-cycles. This number is rather large, and suggests that a voice coil of proper design in a loudspeaker is capable of experiencing an enormous number of cycles. We might consider a more typical case, where the loudspeaker is used only 4-hours per day. This reduces the ten-year cycle count to only 42,048,000,000-cycles.

From some data collected from my E-TEC engine, I know that 66-percent of the time the engine RPM is about 1,000. And 34-percent of the time the engine RPM is about 4,000. This gives an average RPM of (0.66 x 1000) + (0.34 x 4000) or roughly 2,000-RPM. This implies that an E-TEC injector will cycle on average 2,000-times per minute, or 120,000-times per hour. We can now compute how many hours an E-TEC engine has to run (at our average speed of 2,000-RPM) before one of its injectors accumulates 1,000,000-cycles: 8.33-hours. Let us say an E-TEC is run for 100-hours a year, taking ten years to reach 1,000-hours of operation. An E-TEC engine with 1,000-hours of operation will on average have 120,000,000 cycles on its injectors.

Compared to a loudspeaker voice coil, we see that an E-TEC injector voice coil does not experience as many cycles in a ten year period of typical use. The ratio is about 350:1. We do see that the average power in the E-TEC injector is substantially higher, but the E-TEC voice coil has the benefit of liquid cooling.

It is the nature of voice coils that they move in their operation, and, since they must have electrical current flowing in them, some sort of flexible connection to a source of current is needed. The flexible conductor used to make this connection is going to be subjected to a lot of flexing. As we see in the case of a loudspeaker, perhaps billions of flexes of the conductor are needed without failure of the conductor. In the E-TEC injector one of the reported modes of failure has been from loss of electrical connection in the flexible connections. We see that in comparison to a loudspeaker, in an E-TEC there are fewer cycles of flexing of the conductor. However, since the conductor is immersed in liquid, there is more resistance to movement of the conductor than in air, as we find in the loudspeaker. This could produce more stress on the flexible conductors in an E-TEC.

There has been some anecdotal mention that certain E-TEC injectors were prone to failure of the flexible conductors due to an error in their assembly. As I recall, this was reported to have been traced to one individual in the assembly process performing a procedure incorrectly; this ultimately reduced the life span of those injectors. This error was discovered and corrected.

Because of the high amount of electrical energy used in the E-TEC injector, heat is produced. Of course, there is also heat being produced in the combustion chamber, and some of this may flow into the injector assembly. The cooling of the injector by circulation of fuel seems necessary to carry away this excess heat. Too much heat as been anecdotally reported as contributing too E-TEC injector failures.

These observations are entirely my own and based on the assumptions presented. I would be interested to hear any comments about those assumptions. The approximations for electrical power and for duty cycle are rather unscientific, and are based only a rough guess. Better estimates are welcome.

Post New Topic  Post Reply
Hop to:


Contact Us | RETURN to ContinuousWave Top Page

Powered by: Ultimate Bulletin Board, Freeware Version 2000
Purchase our Licensed Version- which adds many more features!
© Infopop Corporation (formerly Madrona Park, Inc.), 1998 - 2000.