4.3.2 – Types of Brush Cutter Engines

2 Stroke or 4 Stroke Engines and future Electric Motors

First I will look at the two petrol powered motors and then get onto the electric motor. Electric power was the first power plant used in the very first weed eaters and will become the main power source for heavy duty weed wacker’s in the future as battery technology gets better. 

Brush Cutter 2 & 4 Stroke, Front View (600x331)

If you do not know the difference between a 2 and 4 stroke engine, this short video will explain this to you. Although it is related to water craft, this video basically says it all.

 

2 Stroke Engine

As you can see in the pictures below, they are a very simple engine with few moving parts and not a lot to go wrong. They produce higher power for the same weight and if you are using a brushcutter for long periods of time, weight can be an important consideration. However they are smokier, smellier, noisier, and more polluting. You are required to mix fuels and if you get it wrong you could seize the engine.2 Stroke Brush Cutter Engine (600x583)I have tried to find power and torque curves for the most common brands of 2 stroke brush cutters, with no luck. I even went to Husqvarna, who gives the most information about their brush cutters and they could not supply one. Below is the only torque & power curve I could find from a larger 2 stroke engine, but the torque curves are similar for all small 2 strokes. The typical 2 stroke’s maximum power is at or near its maximum revs and the torque band is also very narrow, making running at lower revs more difficult.

2 stroke torque curve

One of the biggest disadvantages of a two stroke engine, is the poorer fuel economy and high noise level from the exhaust. This is because about 25 to 30% of the explosion energy of the power stroke is used to expel the exhaust gasses. This also makes it very complicated to design an effective muffler that does not serious restrict exhaust flow, within the weight and size confinements required for a brush cutter or chainsaw. With a motor bike, it is easier to design a well tuned muffler that can work with the engine to help suck out some of the exhaust gasses and then reflect a pressure pulse back to block the inlet gasses escaping into the muffler before the piston seals up the vent. A well tuned muffler can considerably increase the fuel economy & power of a two stroke engine, sadly it is very difficult to fit a well tuned muffler that is almost as large as the engine itself, into the size and weight confinements of a chainsaw or brush cutter. So to keep the power up, your ears must be sacrificed. 

This also explains why 2 strokes engines found in a brush cutters are not twice a powerful as a 4 stroke engines. When you think they should be, since they have a power stroke every revolution and so in theory should be twice the powerful. In fact, generally they are only about 20 to 40% more power for the same engine size as a 4 stroke, the rest of that energy comes out as noise. Want more power, than you are going to have to hang one of these babies off the end of your 2 stroke brush cutter;

Even with the ability of a motor bike to accomodate the weight and size of a well tuned muffler, I cannot think of one brand of road motor bike that still uses a 2 stroke engine, I think that says it all.

Unlike chainsaws that tend to be run intermittently, brush cutter, especially 2 strokes tend to be run flat out for long periods of time. So those with large properties and commercial operators, are exposing their ears to high noise levels for long periods of time. Doing so will eventually lead to ringing in the ears, which I can assure you is not a pleasant way to spend your later years. Unfortunately all the earmuffs used with helmets, are mediocre at best, so I recommend also using ear plugs as well as the helmet earmuffs.

4 Stroke Engine

As you can see in the pictures below, they are a lot more complicated an engine with a lot more moving parts and for this reason are a heavier and more expensive than a 2 stroke engines of the same power. Even though they are heaver for the same power you would get from a 2 stroke, they do have more torque over a wider speed range, so you do not have to run it flat out all the time. I am afraid to say I am very biased towards the Honda 4 stroke engine, I like not having to mess around with mixing fuels, I like the lower noise, the fuel economy, less smoke and smell. Below is the torque & power curves supplied by Honda, Unfortunately it cannot be properly compared with the 2 stroke graph, because it only shows the curve between 4000 and 6000 RPM. It is a shame this graph does not show the torque curve at lower revs, but if you can visualise the lines continuing on and then compare this with the 2 stroke graph above, you should be able to visualise the difference.Honda GX35 Engine Torque Curve - 600wBut best of all I like the easy starting. For me, all of these advantages more than make up for the extra weight and if you have a good harness weight is not so much of an issue.4 Stroke Brush Cutter Engine (600x572)Honda has a saying, that there engines start first time every time and this has proven to be true for me and after 15 years my first Honda still starts pretty much first time every time.

I have seen a number of comments online, where people in their ignorance, say that because it has an oil sump, that if you turn it on its side or back if you are pruning a branch, that the engine will starve of oil and seize. This could not be further from the truth, so let me explain how this beautiful little Honda engine is lubricated.

At the back of the engine, there is a cylindrical sump that holds the oil separate from the crank case.Honda GX35 Lubrication System(600x358)When the engine is running, the two rotary slinger arms froth up the oil to a mist, which is drawn up into the overhead cam compartment via the toothed belt duct where it lubricates all the moving parts there. This mist is then ducted down the side of the engine to the crank case where it lubricates all the main and big end bearings there before finding its way back to the sump. The oil is moved around the engine by the pumping action of the underside of the piston and a one way valve at the bottom of the crank case pumps the mist back up into the remote oil sump where it is frothed up and recirculated again. Honda engine oil return pipe (600x388)Honda-GX35cc Engine-lifetime-timing-belt

Below is a very professional video of servicing the Honda GX25 overhead cam engine, the GX35 would be very similar.

 

I have also seen a number of comments in forums where people state that the 4 stroke engine is not as reliable, because there is a lot more moving parts and more to go wrong. It is for this same reason that these people still drive 2 stroke cars, because they have less moving parts, less to go wrong and so are more reliable. Sounds logical to me, so where can I buy one?

The genuine Honda engine comes with a three year domestic and commercial warranty, (at least here in Australia) so if Honda is confident enough to give it a three year commercial warranty, you can be sure you will get a long trouble free life from it in a domestic situation. Of cause no engine is totally trouble free and on the occasion there can be some parts that fail prematurely. I personally have never experience any problems with my genuine Honda engine brush cutters and if Husqvarna & Maruyama can use a Honda engine in their 4 stroke models then you have to know that these engines are good for the long hall.
Personally, I think that this is the best small engine that you could fit to a brush cutter. You have to give credit to Honda for designing such a beautiful engine, I is a shame Honda are so unless at marketing it. I have had feedback from customers, who have gone to buy a Honda brush cutter after reading through these pages and have been talked out of not only buying a Honda brush cutter, but have also been talked into buying a bicycle handle brush cutter, by dealers with little practicable experience. And this is from dealer who’s main brand is Honda, but also carry other brands.
I have had one customer from the states, that this happened to and he went back and traded his brand new other brand brush cutter for a Honda. When I talked to him directly, he confirmed what I have been saying, that salesman selling brush cutters, really have no idea about the products they are selling.
If you have experienced this problem, please let me know, it would be interesting to know if this is widespread or just an odd occurrence?
As far as reliability is concerned, I am sure the reason why there are so many rumours going around, that Honda engines don’t last because they are so complicated, is because there are so many cheap Chinese copies of Honda engines floating around. I would like to say there are Honda engines and then there are Chinese copy Honda engines and there the similarity ends. I should know, I had been loured by the cheap prices on eBay and brought one, silly me.
Even though they are an exact copy of the real Honda engine, the materials used to make it are not, especially when it comes to anything made of rubber. Why not to buy Chinese copy Honda engine 1Why not to buy Chinese copy Honda engine 2This one started leaking oil within the first 5 hours and I think I was lucky to get 30 hours out of it before I was topping up the oil about every half hour. The rubber petrol tank isolators fell apart very quickly as well and the tank ended up rattling round, but at least it did not fall out, thanks to the Honda design.Why not to buy Chinese copy Honda engine 3Why not to buy Chinese copy Honda engine 4It eventually got to the point that I thought, dam it, I will run it out of oil so it seizes and then throw it out. But to give credit to the Honda design, it did not seize, it just ended up running slower and slower to the point it did not have the power to cut any grass, but is did not seize. I put some more oil in it and it was as good as new, for about half an hour that is. These Chinese copy Honda engines look exactly like the real thing, so unless you actually buy from Honda, there is no guarantee that you are getting a real Honda engine.
Of cause nothing is perfect and there is a fault with the Honda engine that I had blamed on the Chinese copy, but I had to take that back because the Chinese had copied the real Honda fault to a tee. One of the best thing about the Honda design, was that you could run the brush cutter any way you want, on either side and even upside down right to the very last drop of petrol. Unfortunately the gumby who designed the fuel tank for the second generation overhead cam model, only designed it for the more common bicycle handle model. The connection between the right and left side of the petrol tank is too small to let the fuel filter pick up move freely from one chamber to the other, so instead of fixing the tank they just shortened and used a stiffer hose so the pick up sat in the middle of the tank. After all, you can’t see it and no one will notice.

If you want to cut on the side, which with a Loop hand brush cutter, you will want to do a lot of side cutting. Unfortunately, if the tank is less than half full, it will starve of fuel and stops unless you quickly straighten it up for a short time to suck some more fuel and then back on its side again. The first generation Honda that I had, did not have this problem. The pick up filter was attached to a long very soft flexible hose that was able to always move to the lowest point in the tank and suck the very last drop of petrol no mater which way up the engine is sitting.

I am please to report that Honda have addressed this issue, by putting a smaller fuel filter pickup on their latest models, as you can see with the 35cc and 25cc on the left below. Good for them.

I am also a little disappointed that the second generation, overhead cam engine seems to my ear, a little noisier than the first generation engines. The engines of brush cutters are very close to the body and the noise from the exhaust not only has an effect on the ears, but also the body. The effect on the body becomes more noticeable as the engines gets larger, which I found out after using the 92cc monster engine on the cheap Chinese brush cutter. After a couple hours of use with this beast, I felt nauseous in the stomach and this was just from engine noise.Honda engins with small muffler(600x516)To be fair to Honda though, this is still the quietest brush cutter engine on the market, but for a little extra weight it could be a lot quieter. I personally, would happily carry the extra weight of a more efficient muffler that reduced the noise volume even more. I know that my ears and neighbours would appreciate it and so would commercial operators who spent hours on end hanging onto these machines.

I would also appreciate any feedback on this as well. If you happen to visit your Honda dealer and mention the tank issue and perhaps that it would be great it it was quieter, were the Honda dealers helpful, or did they just fob you off. Please let me know, so I can keep you all informed. Petrol engines still have a few years left before electric power takes over. It would be nice if Honda released mark 3 version, addressing the fuel tank issue and improving the muffler to make it that much quieter again.

Other 4 Stroke Models

When I first released this page, the only other 4 stroke engine I could find any information on, was the Stihl 4-Mix, which didn’t seem to have much of an uptake. But those were the days when everyone believed the 4 stroke engines were more complicated and so more unreliable. Since then more and more manufactueres are coming out with variations of the 4 stroke engine, which is good news.

Husqvarna have a 4 stroke model, they use the 25cc Honda 4 stroke engine, but with a different outside plastic housings to match the look of their brush cutter range. Good for them, why spend huge amounts of money trying to develop your own engine, when a brilliant one already exists.

Hopefully they will add the 35cc Honda engine to their range in the future. I have purchased one of these machines and will do a review of it in the near future.

Subaru-Robin show online that they make a 25cc and 35cc 4-stroke engines with a similar lubrication system to the Honda engines.This engine has an overhead valve push rod, similar to the first generation Honda Engines and with a similar power rating to the first generation Honda engines.Since this Robin Engine are owned by Subaru, wouldn’t it be nice if they could make a small 30 to 40cc two cylinder boxer engine. This would give the power of a 2 stroke engine, as there would be a firing stroke every revolution. But with more torque than any 2 or 4 stroke single cylinder engine. More importantly, the biggest benefit would be the much lower vibration that you would get from a horizontally opposed twin cylinder engine. As the reciprocating vibration from the two pistons cancel out each other, allowing for a very low vibration at all speeds.

You could also potentially very easily supercharge this engine, if the compressed air from the two downward moving pistons in the crank case were forced into one piston chamber as it was drawing air in. You would be forcing the air/fuel mixture in under pressure as a supercharger would, which would considerably boost engine power. You could potentially end up with a light weight 25cc engine producing the power of a 40 to 50cc engine, NOW we are talking real power with very little vibration. Sadly I do not see anyone investing the time or money in developing such an engine, as I think that by early 2020’s the main power source for garden tools will be all electric.

Well it seems like Shindaiwa, may have read what I have said here and come up with a similar innovative way of supercharging a single cylinder engine that looks very interesting. SEE BELOW FOR DETAILS. All they need to do now is turn their new 25cc Hybrid4 engine into a twin cylinder boxer engine and you would have the perfect high powered low vibration engine for heavy duty brush cutters.

Makita offer 3 sizes of 4 stroke engines, 25cc, 33cc, & 43cc and from what I can determine, they are re-badged Subaru-Robin engines.

Below is the 43cc engine, which unfortunately is only available as a bike hand brush cutter.

Below is the lubrication system for the Makita 4 stroke engine.

I have not tried one of these brush cutters, so I cannot comment on them, but I imagine that they should be OK. If you have had good or bad experience with these brush cutters, please give me some feedback, so that I can share with everyone.

Maruyama also offer a 4-stroke Honda powered range of brush cutters, (good for them) but there is one caveat. They do not offer this option to the US, Australia and New Zealand distributers. Maruyama - Honda powered 4-stroke brush cuttersNot sure if this has anything to do with licensing agreements with different countries, or they think there is no demand for 4-strokes in these countries. But if you are a Maruyama and 4-stroke fan and looking at getting a new brush cutter, insist that they get one in for you. If they won’t entertain this idea, tell them that you will import one from a country that does sell 4-stroke Maruyama brush cutters.

Stihl also make what they call a 36cc 4-MIX engine, which is a 4-stroke engine without the typical oil lubrication system. Instead they use the standard 2-stroke oil/fuel mixture, to lubricate the engine. This solves some of the issues of how to lubricate the engine on its side and upside down, but the down side is, like a 2-stroke you are still burning oil.

This is what Stihl say about their engine – (The award-winning STIHL four-stroke engine that runs on a petrol-oil mix. The STIHL 4-MIX engine thus combines the advantages of a 2-stroke and a 4-stroke. As well has having plenty of lugging power and perceptibly higher torque, the 4-MIX engine is also convincing in terms of lower emissions, low maintenance and a pleasant soundSort of confirms what I have been saying about 4-stroke engines all along, especially the low maintenance. How can that be, it has a lot more moving parts, so more to go wrong. Well that is what the people, that are still driving around in a 2-stroke cars say. I have not used one of these myself, but I have had good feedback about them from a couple of customers. I have heard that Stihl have discontinued the loop handle version with this 4 strokes engine, in some countries. I will try and get my hands on one of these machines to do a review in the future. From what I have been able to find out at the moment, it looks like about a third of the fuel-air mix is routed through the engine to lubricate it.

 

Shindaiwa, Echo have finally released their, what they call Hybride-4 Engine. But as of April 2018 is still not available in Australia as yet. It is basically very similar to the Stihl 4 Mix engine, but with some interesting features that, I think will make it much better than the Stihl engine.  Going by the American site, it comes in a 25cc and 34cc sizes.

What makes the Hybrid 4® engine different to the Stihl 4 Mix? With the 4-Mix, the fuel-air mix is sucked in under a vacuum. With the Hybrid-4, all the gas/oil fuel mixture is drawn into the crankcase on the piston upstroke. Then, as the piston comes down, the fuel-air mix is compressed by the downward moving piston and pumped through another one-way reed valve into the Power Boost Chamber beneath the crankcase. As with all four-stroke engines, there are two down-strokes to every cycle and it is the second down-stroke that pressurises the fuel mix within the Power Boost Chamber and in affect acting as a super-charger. The result is similar to what I said about supercharging, the two cylinder boxer engine above. Brilliant Idea, good for them for striving to make such an improvement.

This should provide a much greater lower end torque from this engine, which for me makes it a very interesting engine that I will have to try. I had hoped that the power of this engine would be more than this 1.0 kw or 1.4 hp which is exactly the same as the Honda GX35 engine. As yet I have not been able to find any torque figures for this engine and probably never will find out his information, but in theory it should be greater than the Honda. Shindaiwa does have a good reputation, especially with the commercial operators. But like Stihl, their web site, brochures and operating manuals, do not tell you the engine power, or torque of the engine, or the vibration levels at the handle bars. I will have to drop into a local Shindaiwa dealer, to see if they even know.

When they come available in Australia, I will have to get one to do a review of it.

Engine Torque
I had one customer send my an email with a picture of the torque curve for his 2 stroke brush cutter engine. (when I find that email again, I will add the brand name and torque image here) The curve showed that this 2 stroke engine had a very flat torque curve similar to the 4 stroke Honda engine and I wondered how this could be. Other than the fact that I knew 4 stroke engines had a flatter torque over a wide range of revs, compared to a 2 stroke engine, I had not give to much thought as to why this was. So after a lot of research, I now know why and will try to explain this in very simple terms for you.
Torque is the measure of power required to turn a shaft and is measured in either foot/pounds or newton/meters.
There is a huge amount of information already on line and on Youtube explaining torque, so I will only explain how it affects brush cutter engines.
Torque is a product of two main things, the length of travel of the petrol exploding power stroke and the weight of the flywheel.
2 strokes do not have any valves, but relies on openings in the side of the cylinder wall to allow the air-fuel mix to enter and hot exhaust to exit. This means that almost half of the power stroke is wasted in the process of expelling the hot exhaust gases out of the cylinder as you can see below.
2 strokes engines have a power stroke every revolution, which means more power for the size and weight of the engine. This also means that this engine only requires a small flywheel to keep the engine turning over at idling revs and this also keeps the weight of the engine down.
4 stroke engines, have valves to let the air-fuel mixture in and the hot exhaust gases out, which means that this engine gets to use the full travel of the power stroke to provide power for a longer period.
4 stroke engines, have a power stroke every two revolutions of the engine, and so must have a much heavier flywheel to keep the engine turning over for two revolutions and have enough reserve energy to compress the gases for the next combustion cycle.

It is the weight of the flywheel that helps give the 4 stroke a flatter torque curve, but you could also add a heavier flywheel to a 2 stroke and give it a flatter torque curve, as mentioned by the person who sent me the email with flat curve of his brush cutter engine. A heavier flywheel also absorbed torsional vibration, so even though a 4 stroke only fires every second rotation, it tends to have less vibration.

It is this same reason why diesels engines have so much torque, yes there is a higher energy density from diesel fuel, but it is also the extra heavy flywheel that is needed to crank the engine over to the much higher compression to ignite the diesel fuel. The problem with a single cylinder engine, is that it needs a large flywheel to keep the engine turning over and running smoothly. The more cylinders an engine has, the more power pulses per revolution and the less the need for for a heavy flywheel. Get to a twelve cylinder engine and you do not need a flywheel, there is enough weight in the crank shaft and the six power strokes per revolution to keep the engine during over smoothly.

This brings me to something I see a lot of on Youtube, especially those coming out of the United States. Those people who are using loop hand brush cutters, rarely seem to use a harness and have to carry the full weight of the brush cutter in their hands. You even see people using bicycle handle brush cutters without a harness, I don’t know how they manage it, but they do. It seems like some brush cutter manufacturers have jumped onto this and spout on how light there brush cutters are. The simplest way to reduce the weight of a single cylinder engine is to reduce the weight of the flywheel and increase the idling revs for it to keep running. The limit to this, is the speed at which you can pull the engine over with the pull cord. If the engines minimum idle speed is faster than the pull cord speed, than you will not be able to start the engine. This is often the reason why 2 stroke engines, as they get older and no longer in perfect tune, become much harder to start. The other down side to a lighter engine, is the increased vibration you are subjecting your hand and body to, even at higher revs. The lighter you make the a single cylinder brush cutter engine, the more your body become the absorber of the vibration produced by that engine.

Electric Motors

As of 2017, most major manufacturers of brush cutters have a battery powered electric weed wacker. some have the motor and battery at the end of a conventional brush cutter shaft driving and standard gearhead. But the main trend is for a direct drive high power brushless DC motor. At his point in time, I have not seen a direct drive electric weed eater that is heavy enough to carry a metal blade, but this is only a matter of time before one comes out.

If you have any questions about types of brush cutter engines, brushcutters in general, or would like to share with me what you do and especially don’t like about your brush cutter, please email it to me. The more useful information I can add to this page, the more it will give future buyers of brush cutters the right information to make the right purchase decisions that best suits their needs.

If you do email me, I am afraid that I no longer have time to reply in a timely manor, if at all, but any feedback could help add to the knowledge available on these pages.

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