2 stroke v 4 stroke

The more Strokes the better

Rick Huckstepp uses good old Aussie ingenuity to rig up a test bed for comparing a two-stroke outboard engine against its four-stroke counterpart. Here are his results...

Figures bandied about late last year indicate that there is in excess of 750,000 two-stroke engines in operation in this country.

If one browses the pages of TrailerBoat they will find  plenty of advertising encouraging the move to four-strokes, while one company promotes a brand of two-stroke that they state is, if not more, as environmentally friendly than all the others. That particular company does not have a four-stroke outboard motor in its stable. It shares with the many four-stroke outboard engines the VELS 3-star rating in relation to exhaust emissions and is direct injection two-stroke technology compared with carburetted two-stroke systems, the latter which for the purpose of this article I will refer to as 'old technology'.

There are still a lot of new two-stroke outboard motors for sale in this country and due to the amount of time this style of engine has been on the market, are relatively cheap to manufacture and therefore the savings are passed on to the end user. In many cases, too, those two-strokes are lighter in weight than their four-stroke counterpart in the same brand.

In many cases the hip pocket dictates what ends up going on the back of our boats, but is there a bigger picture to this?

For those putting many, many hours on their engines, they need to look at the longevity of old technology two-stroke motors against modern-day engines with new innovations.

TWO-STROKE TERRITORY
In a past life running inland and coastal fishing charters in the Northern Territory, there was only one engine type available to me and that was carburetted two-stroke. I employed them to 75hp with tiller controls and racked up on average about 1200 hours per year per boat.

A lot of that time was spent trolling for barra and pelagics, and with fuel mixes set rather than variable as in modern direct injected two-strokes we see nowadays, the constant over-oiling created all sorts of issues.

Smoke was one of them and the other was the accumulation of oil and residue round the piston rings which eventually nipped them up causing a seizure. This amounted to a lifespan on average per engine of between 650 and 850 hours.

I had one engine that spent all its life on freshwater and it realised a grand old age of 1200 hours before it fell over and had to be replaced. I still cannot work out what the fresh had over the salt, but anyway... what I would have given to have had a smokefree and quieter environment. My lungs would be better off and I would not have a 30 per cent reduction in hearing on my left side and 25 per cent on the right - a legacy of hanging on to a tiller for 12 hours a day!

Subsequently, I moved into four-stroke engines and my last 90hp had 3000 hours on it before I traded it. The purchaser was a marine biologist in Townsville who asked for a compression test before the transaction. This was done by a certified dealer and the result was 220psi in each cylinder. Now that's pretty good when one takes into account I received good money for an engine that lasted three times the life of a carburetted two-stroke. Did it cost me three times more than old technology? No way! I realised then the advantages of modern technology, not to mention the amount of money I saved with better economy and undoubtedly, a slowing down of the inherent hearing loss!

Running a definitive comparison on two types of engines is difficult due to the cost and time factor, and getting two engines of relatively similar age onto the same boat carrying the same payload.

TIME TO COMPARE
Recently we had the opportunity to run comparison tests on the same boat with a two-stroke engine and then a short time later, a four-stroke model.

Both engines were Yamaha in the 40hp class. The two-stroke had approximately three hours running on its side and weighed in at 93kg, and the four-stroke had the same running time and weighed 96.9kg. With another seven hours to the end of the nominated run-in period, one might expect some 'loosening up' of the engine components, but the results to the test would most likely be negligible. Both outboard motors were fitted with an 11-3/8 x 12-inch propeller.

Assembling the most technical equipment available to the general public, we conducted the tests on the Brisbane River.

While the calibration of the test equipment might not withstand the rigorous checks of the Weights and Measures authorities, it is to some extent, what is generally used by us boaters every day on the water. Also, the identical equipment was used to test both engines.

Firstly, on the day of the two-stroke test, the wind was from the east at 10 to 15kmh and blowing directly up the river with an incoming tide that had the test boat, a Webster Twinfisher, drifting at 0.7kmh on average when dead in the water.

The four-stroke test day had wind of 15 to 20kmh from the east and in the test area used for the two-stroke test, the run-out tide against the wind created chop to 0.75m making it difficult to get a reasonable average of figures.

The Webster was relocated around a bend in the river where we still had the wind at 15kmh, but beam on and a run-out tide. Drifting dead in the water at the second test site had the GPS recording a speed over ground on average at 0.9kmh.

On both engines, the information was recorded running in opposite directions and then averaged out to arrive at the final result.

We ran both engines at the same rpm and recorded the speed in kmh, the kilometres to one litre of fuel used, the litres per hour used, the noise levels in decibels, and a shakedown of their acceleration performance from a standing start over 10 seconds.

The rpm was derived from a digital spark induction tachometer that was installed on a sparkplug lead of each engine and was reset to reflect the spark regularity of each type of engine.

The fuel statistics were derived from a paddle wheel fuel-flow meter fitted in line to a 22lt tote tank and was incapable of registering less than 2lt/h (litres per hour) consumption.

Ambient sound levels were recorded with a Dick Smith Digital Sound Level Meter. This was held with the detector end directed at the front of the engine cowl from a position at the tiller handle grip which, on both engines, was the same length away from the powerhead. One must take into account here the different wind levels of each day and the direction it blew in relation to the detector end of the sound level meter. It would have some effect on the end result, but that extent cannot be quantified outside of a controlled environment such as a test room.

THE VBOX
Speed and distance for the acceleration test was recorded into a VBox testing apparatus from which details were downloaded to a hard drive.

While the two-stroke engine was fitted and performed correctly, the four-stroke appeared to be mounted one bolt hole too high and experienced some minor aeration at the propeller when jumping across the top of chop. On smoother waters of the test site, this aeration disappeared and the engine operated normally without over-revving.

With Glenn Gibson from Yamaha at the tiller, I sat on the same thwart seat to observe the readings as they were recorded. The fuel tank and all equipment were at our feet and no extra weight was forward in the boat. Collectively, Glenn and I weigh 178kg. Where are you Jenny Craig?

Both engines exhibited a similar power-band until 4000rpm was reached and the four-stroke inched away from the two-stroke as far as speed for the rpm is concerned. Once they reach full throttle, though, you will see parity between the two.

The litres-per-hour tests showed the two-stroke consistently using more fuel than the four-stroke, although we thought the difference would be more marked across the entire rpm range. This usage does not become exaggerated until 4500rpm and then the gap widens considerably, ending at full throttle when the two-stroke is using roughly 5lt/h more than the four-stroke.

In the 10-second acceleration challenge, the two-stroke pulled away from the four at the two-second mark and leads in speed until seven seconds into the test, by which time the four-stroke reached the same level.

Holeshot is usually around that two to three-second mark and - while quick holeshot is critical when using a skiboat to alleviate strain on the skiers arms and in big trailerboats which gobble fuel at an alarming rate during this stressful time on the engine - in a boat of this type and its intended purpose, it's not as vital to be on the plane as quickly. If using boats on bar crossings, though, it is handy to have that low-down punch to stay on the back of swells as you ride them home.

The graph showing kilometres to one litre used (km/lt) indicates a wide gap at the beginning.

This large variance between the two and four-stroke comes about because, at idle rpm when testing the four-stroke, we were travelling faster at an average speed of 3.9kmh compared to the two-stroke, also idling and travelling 2.5kmh, a variance of 1.4kmh. Fuel consumption on the four-stroke at this stage of the test was not measurable, being under 2lt/h and we could not get it to record consumption until 2000rpm, at which time it recorded 2.15lt/h.

At 1000rpm the two-stroke was using 2.55lt/h. This gave us a staggering 6.55km/lt increase in distance which would pay dividends if you were trolling for a considerable period of time in your style of fishing and boating.

At idle rpm this is the typical case; four-strokes using considerably less than two-stroke motors.

Where fuel economy matters most is at planing speeds of above 3200rpm to wide open throttle, and you will see the graph somewhat closes the gap, but the four-stroke always holds the edge. In this case, at 4000rpm we reached optimum km/lt on both motors, with the four-stroke leading by 0.71km/lt. At this rpm the four-stroke will cruise at 28.15kmh using 7.5lt/h while the two-stoke consumed 8.95lt/h at the same rpm and a speed of 27.25kmh.

While the lack of smoke from the four-stroke engine is welcomed by all, one has to look at the style of boating the motor will be used for and other factors such as the price difference between the two technologies, the amount of hours/km it will be used for, the weight differences, and the power requirements. And then there is the longevity of life out of each and the resale value when you want to trade or sell.

While it was opportune to test these two engines as we did, the most common horsepower engine sold in Australia is 15hp and we would expect to see the various factors of a four-stroke and a two-stroke farther apart. That remains to be seen.