Why did the Mazda RX-8 Engines Start Failing
In this article I intend to continue the story to give you a background to the platform and to illustrate why engines swaps have been popular. As with all of this information, it comes from direct experience or information I have collected over the years.
Why did the Mazda RX-8 Engines Start Failing? From what I have seen, the reasons behind the failures of Mazda RX-8 engines have spurred many theories. What follows is a compilation of my understanding based on the knowledge I accumulated during my time with rotary engines.
Primarily, the issues revolved around inadequate lubrication and the accumulation of carbon deposits internally. As for the root causes, there is quite a list to delve into – so brace yourself!
Oh and if you are interested in understand what signs to look for when the engine is failing, check out: Signs your RX-8 Rotary Wankel Engine is Failing
Traditional engine lubrication
To understand more about the issue, it’s important to take a quick look at how a traditional engine works. Modern four-stroke engines (found in the majority of gasoline/petrol-based cars) can trace their heritage back to the mid-1800s, originating from the work of traveling salesman Nikolaus August Otto. They have come a long way since then, but oil has remained one of the primary methods for cooling and reducing wear during that time.
In a conventional four-stroke piston engine, the rotating assembly is normally lubricated from the sump – a designated reservoir where oil collects (sumps can either be wet or dry). The oiling system in these engines directs oil straight into crankshaft and piston bearings, ensuring a consistent film of oil for the rotating parts to glide on. Additionally, oil is sprayed onto the cylinder walls and piston wristpin from beneath the piston. This layer of oil helps the piston to move up and down with minimal friction. The piston rings play a crucial role in maintaining a barrier between the combustion chamber and oil chamber. While this seal is highly effective, it is not absolutely perfect; a minuscule volume of oil invariably enters the combustion chamber burnt off. Hence a certain degree of oil consumption in piston engines is considered normal (conversely combustion gases can also enter the oil chamber).
Rotary engine lubrication
In contrast to piston engines, rotary engines present a unique challenge when it comes to lubrication. The rotating assembly in a rotary engine cannot be lubricated using traditional methods, since the sealing surface between the engine walls and the rotating components is internal to the combustion chamber. Consequently, oil has to be directly injected into the combustion chamber to facilitate lubrication. As the combustion cycle progresses, this is burnt with the air/fuel mix. For those familiar with two-stroke engines, you might already be able to see where this is going 🙂 Unlike burning pure petrol or gasoline, burning oil results in a higher accumulation of deposits.
Engine Carbon Fouling
Carbon fouling of engines is not a novel issue. Decades ago, concerns about carbon build-up in gasoline/petrol vehicles were commonplace. Fortunately, advancements in engine and fueling technologies have brought significant improvements over the years. The introduction of electronic fuel injection was a revolutionary step, making carbon build-up less prevalent in modern engines. However, there was a slight setback with the advent of direct injection vehicles, but that topic merits its own detailed discussion.
As touched on earlier, the benefits of cleaner combustion seen in modern injection vehicles are somewhat negated in rotary engines. The practice of directly injecting and subsequently burning oil in the combustion chamber poses immediate challenges. Furthermore, achieving a complete burn in a rotary engine is more complex, necessitating two spark plugs per rotor. This complexity introduces yet another cause of increased carbon residues.
Apex Seals and Carbon Deposits: A Problematic Combination
What makes carbon deposits particularly detrimental in rotary engines? The issue primarily centers on where these carbon deposits accumulate. Each rotor has what’s known as Apex seals at the tip of each corner. These seals are mounted on tiny springs, allowing them to adapt to the contour of the rotor housing, ensuring a tight seal. However, any deposits gathering at the rotor tips, where the Apex seals are located, can cause these seals to become jammed, preventing the tips from sliding in and out in in response to thermal expansion.
The consequence of this jamming becomes evident when the engine is hot, making the car difficult to start due to decreased compression. This happens because the distance between the rotor housing and the tip widens too much, given that the Apex seals can not slide out to engage the housing. To counteract this, the engine either has to spin more rapidly (more on this in another article) or allow the engine to cool down to regain compression.
A different challenge arises when the engine has completely cooled. If the carbon deposits hinder the Apex seal from retracting into the rotor, the space between the housing and the seal becomes too tight. This results in quicker wear of the Apex seal, ushering in a vicious cycle that further aggravates the problem.
Oil Volume: A Critical Aspect
Was stuck APEX seals due to carob deposits the only issue? Unfortunately not for the early years. It appears that, especially in the early RX-8 engines, the volume of oil injected was either insufficient or misdirected. In a deviation from the RX-7 engine’s design, the early RX-8 models featured only two oil injectors per rotor, having removed the central injector, likely for emissions considerations. Later models (from 2009-2012, termed “Series 2”), which experienced fewer complications, saw Mazda reintroduce the central oil injector, bringing the total back to three per rotor.
For the earlier “Series 1” cars (from 2004-2008), retrofitting a third injector wasn’t feasible. As a workaround, Mazda introduced updated ECU software. Dealers were then able to reflash the ECU to increase the oil volume dispensed by the two existing injectors. However, this remedy might not have been ideal. Even with the added oil, it might not have dispersed evenly throughout the combustion chamber. In cases where it wasn’t fully combusted, this could potentially contribute to heightened carbon fouling. But, this is speculative.
So here we have early engines with potentially either too little oil entering the combustion chamber for lubrication, or an excess of oil in an attempt to reach parts of the rotating assembly that the third oil injector would have previously lubricated. Not ideal.
Summary
The above material outlines my understanding of the two primary root issues that have caused many RX-8 engines to fail in the field. Unfortunately, the list of factors that could contribute to the above issues and accelerate wear is a long one, which I hope to outline in my next post!
Thanks again for reading!
If you own an RX-8, at some point you will likely have to pressure check the cooling system. More on that in the future. Next time we will dig into the Mazda RX-8 Maintenance Nightmare!