Why is it that even though E1 closed when paired with valve 8 (I4): 1 + 8 = 9. I4 also remained closed instead of open.
Answer.
I understand your confusion, and I appreciate the opportunity to clarify the Rule of Nine and how it applies to pairing valves in a 4-cylinder engine. The key issue here seems to be the relationship between valve positions (open or closed) and how the Rule of Nine determines which valves to check when one is open or closed.
Let’s break this down step-by-step, addressing the specific scenario you described and clarifying why certain valves are open or closed when others are paired to sum to 9.
Recap of the Rule of Nine
The Rule of Nine is a method used to check valve lash in a 4-cylinder engine by identifying which valves are fully closed (and thus ready for lash adjustment) based on which valves are fully open. It relies on the fact that in a 4-stroke engine, the camshaft and crankshaft positions create a predictable relationship between valve states. Specifically:
- When one valve is fully open (at maximum camshaft lift, typically during the intake or exhaust stroke), its paired valve (where the sum of their position numbers equals 9) is fully closed (on the camshaft’s base circle, ready for adjustment).
- The rule assumes a standard valve numbering system and a typical 4-cylinder firing order (e.g., 1-3-4-2).
For a 4-cylinder engine with two valves per cylinder (intake and exhaust), the 8 valves are numbered as follows (based on a common convention):
- Cylinder 1: E1 (valve 1), I1 (valve 2)
- Cylinder 2: E2 (valve 3), I2 (valve 4)
- Cylinder 3: E3 (valve 5), I3 (valve 6)
- Cylinder 4: E4 (valve 7), I4 (valve 8)
The Rule of Nine states that if valve X is fully open, valve (9-X) is fully closed. For example:
- If valve 1 (E1) is open, valve 8 (I4) is closed (1 + 8 = 9).
- If valve 7 (E4) is open, valve 2 (I1) is closed (7 + 2 = 9).
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Your Scenario: Cylinder 1 at TDC Compression
You mentioned the situation where cylinder 1 is at Top Dead Center (TDC) on the compression stroke, meaning:
- E1 (valve 1) and I1 (valve 2) are both fully closed, as both valves are closed during the compression stroke.
- At this point, cylinder 4 is on its exhaust stroke, so E4 (valve 7) is fully open (the exhaust valve opens to expel exhaust gases).
You correctly noted that, per the Rule of Nine:
- If E4 (valve 7) is open, the paired valve is I1 (valve 2) because 7 + 2 = 9, and I1 is indeed closed (as it’s on cylinder 1 at TDC compression).
However, your confusion arises with the pairing of E1 (valve 1) and I4 (valve 8) (1 + 8 = 9). You’re questioning why I4 (valve 8) isn’t open if E1 (valve 1) is closed, since the Rule of Nine suggests that if one valve is open, its paired valve (summing to 9) should be closed, and vice versa. Let’s address this.
Why Isn’t I4 (Valve 8) Open When E1 (Valve 1) Is Closed?
The key to resolving this lies in understanding the engine’s cycle and the specific timing of valve positions at this moment:
- When cylinder 1 is at TDC compression, both E1 (valve 1) and I1 (valve 2) are closed because the piston is at the top, and the cylinder is about to enter the power stroke (no valves are open during compression).
- Simultaneously, cylinder 4 is on its exhaust stroke, meaning E4 (valve 7) is open to allow exhaust gases to exit. However, I4 (valve 8), the intake valve for cylinder 4, is closed during the exhaust stroke because the intake stroke hasn’t occurred yet.
Now, let’s apply the Rule of Nine:
- Since E4 (valve 7) is open (cylinder 4 exhaust stroke), the Rule of Nine pairs it with valve 2 (I1): 7 + 2 = 9. This confirms that I1 (valve 2) is closed, which aligns with cylinder 1 being at TDC compression.
- For E1 (valve 1), which is closed (because cylinder 1 is on compression), the Rule of Nine pairs it with valve 8 (I4): 1 + 8 = 9. Since E1 is closed, the rule implies that I4 (valve 8) should be open—but this seems to conflict with the fact that I4 is closed during cylinder 4’s exhaust stroke.
This apparent contradiction arises because the Rule of Nine is a general guideline that assumes you’re looking for a valve that’s open to identify a closed valve for adjustment. However, in this specific position (cylinder 1 at TDC compression), both E1 and I1 are already closed, and the Rule of Nine is being used to confirm which valves can be checked, not necessarily to dictate that the paired valve must be open.
Resolving the Confusion
The Rule of Nine is typically applied by observing which valve is fully open and then checking the valve that pairs with it (summing to 9) because it will be fully closed. In your scenario:
- You’re starting with E4 (valve 7) open (cylinder 4 on exhaust stroke), which correctly pairs with I1 (valve 2) closed (7 + 2 = 9).
- You’re also noting that E1 (valve 1) is closed, and its pair is I4 (valve 8) (1 + 8 = 9). The confusion comes from expecting I4 to be open because E1 is closed.
Here’s why I4 (valve 8) is closed instead of open:
- During cylinder 4’s exhaust stroke, only E4 (valve 7) is open. The intake valve I4 (valve 8) remains closed because the intake stroke for cylinder 4 occurs after the exhaust stroke in the 4-stroke cycle (intake, compression, power, exhaust).
- The Rule of Nine doesn’t mean that the paired valve must be open if the other is closed—it’s primarily used to find a closed valve when you know one is open. In this case, since E1 (valve 1) is closed, the rule suggests checking I4 (valve 8), but I4 happens to be closed as well because of the engine’s timing.
At this point (cylinder 1 at TDC compression):
- Valves that are closed and can be checked for lash include E1 (valve 1), I1 (valve 2), and I4 (valve 8).
- Valves that are open include E4 (valve 7) (and possibly others, depending on the exact camshaft position).
The Rule of Nine confirms that when E4 (valve 7) is open, I1 (valve 2) is closed (7 + 2 = 9), which is true. For E1 (valve 1), which is closed, the rule pairs it with I4 (valve 8), but since I4 is also closed in this position, you can check it as well.
Simplifying the Process
To avoid confusion, focus on using the Rule of Nine to identify closed valves for adjustment by finding open valves:
1. Rotate the crankshaft to cylinder 1 TDC compression.
2. Check which valves are closed directly (e.g., E1 and I1 are closed because cylinder 1 is on compression).
3. Identify which valve is open elsewhere (e.g., E4 is open because cylinder 4 is on exhaust).
4. Use the Rule of Nine to confirm additional closed valves:
- E4 (valve 7) open → check I1 (valve 2) (7 + 2 = 9).
- Since E1 (valve 1) is closed, its pair I4 (valve 8) (1 + 8 = 9) is likely closed in this position, so you can check it too.
5. Rotate the crankshaft 180° and repeat, identifying open valves and their paired closed valves.
In this position, you can check:
- E1 (valve 1): Closed because cylinder 1 is at TDC compression.
- I1 (valve 2): Closed, confirmed by Rule of Nine (paired with E4 open).
- I4 (valve 8): Closed, as cylinder 4 is on exhaust, and its intake valve isn’t open yet.
Why the Rule of Nine Seems Confusing Here
The Rule of Nine assumes you’re starting with an open valve to find a closed valve. When you start with a closed valve (like E1), the rule’s pairing (I4) doesn’t guarantee that I4 is open—it only suggests that I4 is the valve to check if E1 were open. Since E1 is closed and I4 is also closed in this position, it’s a quirk of the engine’s timing. The Rule of Nine is most effective when you:
- Observe an open valve (e.g., E4) and check its paired closed valve (e.g., I1).
- Use the engine’s TDC positions to directly confirm closed valves (like E1 and I1 at cylinder 1 TDC).
To avoid confusion, many mechanics combine the Rule of Nine with a TDC-based method:
- At cylinder 1 TDC compression, check E1, I1 (both closed) and use the Rule of Nine to confirm additional closed valves (e.g., I4 because E4 is open).
- Rotate 180° to cylinder 3 TDC compression, check E3, I3, and use the Rule of Nine for others (e.g., if E2 is open, check I2 because 3 + 6 = 9).
Step-by-Step Example for Clarity
Let’s walk through the position again:
1. Cylinder 1 at TDC Compression:
- Closed valves: E1 (1), I1 (2) (because cylinder 1 is on compression).
- Open valve: E4 (7) (because cylinder 4 is on exhaust).
- Rule of Nine: E4 (7) open → check I1 (2) (7 + 2 = 9). I1 is already closed, so this confirms you can adjust it.
- Also, I4 (8) is closed (cylinder 4’s intake valve is closed during exhaust). Since E1 (1) is closed, the pair I4 (8) (1 + 8 = 9) can be checked if closed, which it is.
2. Rotate 180° to Cylinder 3 at TDC Compression:
- Closed valves: E3 (5), I3 (6).
- Open valve: E2 (3) (cylinder 2 on exhaust).
- Rule of Nine: E2 (3) open → check I3 (6) (3 + 6 = 9). I3 is closed, so adjust it.
- Check other closed valves (e.g., I2 if E3 is open elsewhere).
This process continues for cylinders 4 and 2, ensuring all 8 valves are checked.
Key Takeaway
The Rule of Nine is a tool to identify closed valves for adjustment by finding an open valve and using the sum-to-9 relationship. In your scenario:
- E4 (7) open → I1 (2) closed (7 + 2 = 9), which is correct.
- E1 (1) closed → I4 (8) is paired (1 + 8 = 9), but I4 is closed (not open) because cylinder 4 is on its exhaust stroke, and its intake valve isn’t active yet.
- The rule doesn’t mean the paired valve must be open if one is closed—it’s a guide to find closed valves for adjustment based on open ones.
To avoid confusion:
- Focus on finding open valves (e.g., E4) and use the Rule of Nine to identify their paired closed valves (e.g., I1).
- When at TDC for a cylinder (e.g., cylinder 1), check its valves (E1, I1) directly, as they’re guaranteed closed, and use the Rule of Nine to confirm additional closed valves (e.g., I4).
Practical Advice
- Use the Service Manual: The Rule of Nine is a general method, but your engine’s manual may provide a specific sequence or valve numbering that simplifies the process.
- Double-Check Valve States: If you’re unsure whether a valve is open or closed, gently press on the rocker arm. If it’s loose (slight play), the valve is closed and ready for lash adjustment.
- Practice the Rotation: Rotate the crankshaft in 180° increments (or as specified) to systematically cover all cylinders and valves.
If you’re still confused or working on a specific engine, let me know the make, model, or year, and I can provide a more tailored explanation or sequence!
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