Can an Engine Run with Low Compression in a Single Cylinder?

When discussing engine performance, one common question is whether an engine can still operate effectively with a low compression in a single cylinder. In this article, we will explore the implications of having diminished compression in a single cylinder and provide insights based on real-world experiences and technical knowledge.

Implications of Low Compression in a Single Cylinder

Low compression in a single cylinder of an engine can indeed allow the vehicle to run, but the performance is often compromised. Unlike a cylinder with perfect compression, the cylinder with low compression will not produce as much power. This can lead to several observable effects:

The engine may run more roughly and less smoothly than usual. Higher RPMs may appear smoother, but the low compression cylinder is not contributing optimally. The throttle opening may need to be increased to compensate for the power loss from the cylinder with low compression. The low compression cylinder continues to consume air and fuel, albeit not as effectively as usual. This inefficiency leads to consume more fuel across all cylinders, reducing overall maximum power and performance when accelerating.

Real-World Examples

One real-world example comes from a personal experience with a Mini. The car was running very rough, and upon inspection, it was determined that the engine was only firing on two of the four cylinders. One cylinder was completely dead, and the other had some signs of ignition but was not contributing significantly. This example proves that an engine can run on low compression in more than one cylinder, although performance will be severely impacted.

Impact on Emissions and Power Output

Despite the reduced performance, an engine with low compression in a single cylinder can still pass emissions tests. The unburned air from the four-stroke cycle can help in the cleaning process of emissions. However, the effect is more pronounced in engines with more cylinders; an eight-cylinder engine would likely show more noticeable issues, while a six-cylinder or a four-cylinder engine would be more affected, as it means the engine has lost a significant portion (25%) of its power.

Testing and Diagnosis

Testing and diagnosing an engine with low compression can be done effectively. For instance, an 83 Toyota pickup which was converted to fuel injection experienced an issue where a leaking fuel injector caused excess fuel to drain into cylinder 1 instead of returning to the fuel tank, washing away oil from the piston rings. After a month, the rebuilt engine had low compression on cylinder 1. The owner drove the vehicle for a couple of years, albeit with noticeable stumbles at lower RPMs.

Typically, all cylinders should have compression within 10 psi of each other and at least 100 psi. More than a 15 psi difference can indicate bigger problems in the long run. Regularly maintaining and diagnosing your engine’s compression is crucial for ensuring optimal performance and longevity.

Conclusion

In conclusion, an engine can run with low compression in a single cylinder, but performance and efficiency will be negatively impacted. Understanding the implications of such an issue and taking proactive steps to address it can help in maintaining your vehicle's performance and reliability.