Comparing the HKS-700T to the Rotax 912 series, (80,
and 100, hp)
By Gerald J. Olenik, pres. Green Sky Adventures, Inc.
Pricing - The big hurdle. Purpose-built aircraft
engines are expensive, that's simply a fact. Actually, engine pricing often
seems to be a floating variable depending on completeness of the package
and a number of other circumstances. But yes, the 700t is certainly in
the dollar ballpark with Rotax 912 80 hp. It really does boil down to what
you get for what you are paying.
Some have asked or commented that the 500 hour recommended TBO is rather small..
The 500 hr TBO of the HKS-700T is an introductory number. For instance its older brother, the 700e began with 500 hours, then with field experience was increased to 800, and now, it is at 1,000. We expect nothing less of the 700t, but 500 is a cautious prediction for now.
When it comes to TBO, the 700e at 1,000 hours has a rebuild cost of $2,100 including standard parts and labor. That's typically less than half the overhaul cost of the 2000 hr 912. So, overhaul cost broken down per hour is probably slightly more favorable with the HKS. Of course, those are real numbers based on history. The 700T is still only projection.
From a performance standpoint the benefit of the turbo is enormous, at any Density Altitude over zero. It even works out better than the 100 hp 912S in some situations. Output of the HKS 700T would be on par with the Rotax 912S (that's the 100 hp version) at about 7,000 ft density altitude, and HKS surpasses the 912S by 10 percent at 10,000 ft density altitude. This is based on a loss of 3 percent per thousand feet.
Comparing to the straight 912, (80 hp), the HKS 700T outperforms it from the sea level up. At 7,000 ft Density Altitude, the HKS produces 24 percent more power than the Rotax 912. On up at 10,000 feet Density Altitude the HKS 700T makes 36 percent more power than the Rotax 912.
Next, compare engine weight. If you hang from a scale, each engine, with all the parts one needs to make it work. the 912 will be mighty close to 160 lb. The HKS 700T 135 lb. Let's just call it a 20 lb difference, though it's really going to be closer to 25 lb.
HKS engines are the personification of fuel economy. The 700-e burns typically between 2.5 and 3.2 gph. These are real street numbers on multiple aircraft with various missions and flight duration. (See my fuel notes from the Quasar Lite) The 700T, with the benefit of electric fuel injection, is reporting fuel burn at least as good as it's 60 hp little brother. That's roughly 1.5 gph less than typical Rotax 912, and perhaps 2.5gph less than the 912 S model.
Combined weight and fuel economy.
Let's plan a 3 hour flight, carrying 1 hour reserve fuel. So, we'll need 4 hours fuel. HKS fuel at 2.6 gph. equals 10.4 gallons, or 62.4 lb. Rotax 912 80 hp fuel at 4 gph. equals 16 gallons, or 96 lb. The Rotax 912 starts out at least 20 lb heavier than the HKS 700-E. Then add the extra 33.6 lb of fuel the Rotax will need for the mission. So apples to apples, the aircraft powered by Rotax 912 begins this mission having to carry almost 54 lb, or 40 percent more than the one powered by HKS-700T. And, that is useless weight.
If you are repowering an existing aircraft, any increase in empty weight must be subtracted from useful load. An aircraft designer may approach this comparison by rationalizing that for every pound of engine weight saved, a pound of airframe weight may be removed from the design. Using our three hour plus reserve mission profile, the weight advantage then becomes 108 lb!
|HKS 700T||ROTAX 912|
|WEIGHT OF 4 HR FUEL||
4 cylinder smoothness with 2 cylinders?
The HKS is very smooth. As smooth as the 912? ...Maybe. Low rpm vibes are inherent with both of these light weight short stroke geared engines. When set up correctly, the 912 would have only a slight advantage at idle power compared to the carbureted HKS 700e. Comparing to the HKS 700T, the HKS may have the edge. Electric fuel injection of the HKS 700t. is far more efficient at any rpm or throttle position. The 700T also has lower compression than it's normally aspirated brother. Both of these features contribute substantially to smooth running at all rpms but make a major improvement at idle and reduced power. Comparing vibration up in the normal operating range of cruise power or above, I think the Rotax 912 would have the edge, with it's four cylinders but only slightly..
Two cylinder reasoning and quantifying smoothness.
Why a 2 cylinder vs 4 cylinder engine design? The HKS incorporates a massive (about 7.5 lb) flywheel to minimize the violence normally associated with 2 cylinder opposed engines. Quite simply, it works. At normal power setting, (in my opinion) the HKS 2 cylinder engine runs as smoothly as any Continental or Lycoming 4 cylinder I've ever flown, and perhaps as smooth as some 6 cylinders. That flywheel may be low tech, but it works. That leaves only two cylinders to which HKS has applied their high tech capabilities where they have created a 4 valve head with oil cooling, a simple pushrod valve train, and predictable integral scavenge pumps for oil transportation. A smart CDI ignition with advance circuitry tops off their high tech stuff. One common concern? What if you loose one cylinder? Loss of one cylinder, (for instance, lets' say one of the carburetors falls off of an HKS 700e) does not have the same effect as losing two cylinders on the same side of a 4 cylinder opposed engine. That's because the massive flywheel. is still there. As far as vibration dampening is concerned, it acts just like two other cylinders would on a 4 cylinder opposed engine. The HKS, in this case would loose slightly less than half power. I have heard reports that the 700e will produce 36 hp on one cylinder. If a carburetor fell off a Rotax 912, violent vibration would render the power of the opposite side cylinders useless. Ditto, if you lost 2 cylinders on the same side of a Continental or Lycoming. Of course, if you lost 2 cylinders of a 2 cylinder engine, it would be infinitely smooth.
HKS has been changing the way people think about high performance since 1973. Since 1998, HKS has established itself as a viable manufacturer of light weight high performance aircraft engines. The HKS 700-T really is the shape of things to come. ...My words. In the words of one of my contemporaries, "its a game changer"