If you’ve ever wondered how petrol lawn mowers work, then you’re not alone. Hopefully this small guide will help you on your way.
The principles behind how a petrol lawn mower works versus an electrical lawn mower are pretty much identical. The main difference, of course, is the source of the power that spins the cutting blades and, in some case, the rear wheels and / or rear roller.
A petrol lawn mower comes supplied with, in most cases, a 4 stroke engine which generate between 3 and 6 horse power. Being 4 stroke means the mower can use standard unleaded petrol that you can buy from your local petrol station. The combustion within the engine then powers the cutting blades of the mower which can either be rotary or cylindrical.
The rotary blade acts like the big hand on a horizontal clock. They rotate clockwise cutting the grass as they turn. In most case, there are 4 blades and they are shaped to produce an updraft of air as they spin like an aircraft propeller. This causes a differential in pressure under the mower which pulls at the grass before cutting it.
The rotating blade mower works much like you’ll see on a combined harvester. One blade will capture and lift the grass and then the next blade in the cylinder will cut it.
Whichever type of cutting blade the mower has, once the grass is cut and collected – either in a collection box or discharged to the side or rear – some mowers that can be bought, such as the McCulloch M46-140RR, come with a rear roller instead of wheels which flattens the grass and creates that great looking striped pattern we all wish for.
How petrol lawn mowers work can be summarised quite neatly in the following 5 steps :
- The operator usually pulls on a starter cord that proceeds to kick start the engine into action. You’ll have seen these many times whether on chainsaws or outboard motors. Many mowers are now being produced with an electrical key start similar to what you’ll find in your car.
- The engine of the petrol lawn mower could have one or two functions depending on the model you are using. Firstly, and most importantly, it will provide the power to the blades to rotate and cut the grass. Secondly, for self-propelled mowers, power will be sent to the rear wheels to drive the mower forward thus negating any need for the operator to push. But remember, the more then engine has to do, the powerful it needs to be.
- There will usually be a throttle on the handlebar that regulates the amount of fuel being fed into the carburettor at any one time. The greater the pull on the throttle, the more fuel the engine receives which increase the revolutions per minute (RPM). As the RPM increases, so does the power output of the engine.
- As already mentioned above, the rotating blade under the mower then captures the grass underneath either by suction or by a cylinder blade “grabbing” the grass and then the blade cuts at the desired height. On most petrol lawn mowers, the operator is able to set the cutting height to one that he chooses – usually between 20mm and 80mm for most home garden mowers. Cutting grass to different heights during the cutting season can contribute to the health of your lawn so careful attention to a mowers cutting heights is important.
- Finally, we need to do something with the grass cuttings. Different mowers offer different options but all will offer a grass collection box of some description. This box sits on the back of the mower and collects the cut grass, compacting it as it does. Some mowers also offer a discharge option where the cuttings are discharges from the mower to the rear but also, sometimes to the side. Finally, there is mulching. Mulching is where the mower cuts the grass cuttings down into really small bits and then reintroduces them to the lawn. These small bits of grass are then broken down by micro-organisms in the soil which help then to fertilize the soil.
So that it. That’s how petrol lawn mowers work. Of course, there is probably a little bit more in the detail, but as a general rundown, that’s your ticket.