Detonation (knock), pre‑ignition, and LSPI are often conflated, yet they differ in timing, triggers, and severity. Detonation is end‑gas autoignition after the spark; pre‑ignition is any unintended ignition before the commanded spark; LSPI is a stochastic pre‑ignition mode peculiar to boosted, low‑speed, direct‑injection gasoline engines. Modern ECUs manage these with knock sensors, ion‑sensing, and closed‑loop timing and fueling. Understanding their mechanisms and the control toolbox explains why engines can run 9.5–11.0:1 compression with 1.2–1.8 bar absolute boost on 95 RON fuel while preserving durability and emissions.
The core distinction is timing and trigger. Detonation occurs after the spark when the remaining “end‑gas” autoignites, sending pressure waves that ring the chamber at its resonant frequency (typically 5–9 kHz for 80–95 mm bores). It’s a pressure‑oscillation problem superimposed on otherwise normal combustion. Pre‑ignition ignites the charge before the commanded spark—via a hot deposit, glowing plug, or oil droplet—so peak pressure arrives too close to, or before, TDC.
LSPI (low‑speed pre‑ignition) is a severe, random pre‑ignition mode at low rpm/high load in turbo GDI engines, often escalating into “superknock.”
Why it matters: mild, intermittent detonation mainly limits spark advance and BMEP; chronic or heavy knock erodes ring lands and head gaskets. Pre‑ignition is far more destructive because the flame starts early, compressing already burning gases as the piston rises. Typical max cylinder pressures in modern turbo SI might be 80–110 bar under normal knock‑limited operation; superknock from LSPI can exceed 150–200 bar, breaking rings, pistons, or rods in a handful of cycles. Mechanisms and boundaries: Detonation risk grows with end‑gas temperature, pressure, and residence time.
High boost, high intake temperature, high compression, and advanced spark push toward the knock limit; high octane (RON/MON), cooled EGR (5–15%), charge cooling (intercoolers, direct injection), and richer mixtures (lambda 0.85–0.90) push it back. A knock‑limited calibration typically runs spark just shy of knock while targeting MBT (maximum brake torque) where possible; at 2,000–4,000 rpm full load, that might be 10–20 crank degrees BTDC for CA50 ~8–12 ATDC depending on bore, turbulence, and fuel. Pre‑ignition differs: it can start at −30 to −10 crank degrees BTDC when a hot spot or droplet ignites the mixture. LSPI is a pre‑ignition subtype promoted by oil‑fuel droplets and deposits in boosted GDI at 1,200–2,500 rpm, high BMEP (e.g., 18–22 bar), and high in‑cylinder temperatures.
Contributing factors include oil with high calcium detergents, top‑land crevice oil, late fuel impingement on piston crown, and high residuals. OEMs mitigate with oil chemistry (more Mg, less Ca), tighter PCV control, piston oil‑jet and ring pack designs that reduce oil in the chamber, multi‑pulse injections that avoid wall wetting, and boost/spark limits in the LSPI zone. Event rates in development are driven below ~1 event per 100,000 cycles at the “hot” corner of the map before release. Sensing and control: Piezo knock sensors (accelerometers) detect the chamber’s ringing in a crank‑angle window shortly after spark—commonly 5–40 degrees ATDC.
The ECU band‑passes around the bore‑specific knock frequency (e.g., 6.5–7.5 kHz for ~86–92 mm bores) and computes a knock index per cylinder. Closed‑loop timing trims spark in small steps (0.5–2.0 degrees/knock event) and slowly relearns advance with no events. When sustained knock is detected, the ECU can enrich (e.g., from lambda 1.00 to 0.88), add cooled EGR, retard cams, or cut boost. Many strategies include an “octane learn” scalar that adapts to fuel quality (e.g., 91–98 RON), intake air temperature, and altitude, biasing the whole spark map.
Because knock sensors hear only after the spark, they are poor at detecting true pre‑ignition onset; by the time ringing appears, damage may already have occurred. Ion‑sensing adds earlier, cylinder‑resolved information. By applying a bias voltage across the spark plug after discharge, the ECU measures ion current proportional to flame front presence and pressure/temperature. Ion‑sensing can estimate combustion phasing (CA10–CA50), detect knock onset within a few crank degrees, and flag abnormally early flames indicative of pre‑ignition—even before the knock sensor rings.
That enables fast responses: immediate spark retard, fuel and boost cut for the affected cylinder, and diagnostic logging. Cylinder pressure sensors offer the best fidelity (direct p‑θ), but cost and durability limit them to development and niche production. Together, knock sensors for robustness and ion‑sensing for immediacy allow closed‑loop control to ride the edge of MBT without crossing into damage. Trade‑offs and calibration: Avoiding knock with rich mixtures raises exhaust temperature and particulate, increasing catalyst and GPF loading; EGR lowers NOx and knock but can slow burn, hurting stability at idle.
Retarding spark cuts peak pressure and knock risk but sacrifices torque and increases BSFC. Boost reduction protects against LSPI but softens low‑rpm response. Hardware changes—smaller bore, higher tumble, 9.5–10.5:1 compression on high‑boost engines, sodium‑filled valves—shift limits but add cost. Implications: Reliable engines separate detonation control (continuous, closed‑loop spark/boost/fueling guided by knock sensors) from rare, high‑severity pre‑ignition/LSPI control (architecture, oil/fuel selection, injection phasing, and fast aborts via ion‑sensing and torque intervention).
Emissions compliance favors cooled EGR and precise phasing over heavy enrichment. Cost pushes toward robust knock sensing and adaptive timing rather than exotic hardware, while drivability demands fast, transparent torque management. The result is broad‑shouldered torque curves with restrained low‑rpm boost, calibrated to maintain knock indices below thresholds and LSPI incidence effectively zero in real‑world use.